property assignment exception javascript

11 Properties: assignment vs. definition

• 11.1.1  Assignment
• 11.1.2  Definition
• 11.2.1  Assigning to a property
• 11.2.2  Defining a property
• 11.3.1  Only definition allows us to create a property with arbitrary attributes
• 11.3.2  The assignment operator does not change properties in prototypes
• 11.3.3  Assignment calls setters, definition doesn’t
• 11.3.4  Inherited read-only properties prevent creating own properties via assignment
• 11.4.1  The properties of an object literal are added via definition
• 11.4.2  The assignment operator = always uses assignment
• 11.4.3  Public class fields are added via definition
• 11.5  Further reading and sources of this chapter

There are two ways of creating or changing a property prop of an object obj :

• Assigning : obj.prop = true
• Defining : Object.defineProperty(obj, '', {value: true})

This chapter explains how they work.

For this chapter, you should be familiar with property attributes and property descriptors. If you aren’t, check out §9 “Property attributes: an introduction” .

11.1 Assignment vs. definition

11.1.1 assignment.

We use the assignment operator = to assign a value value to a property .prop of an object obj :

This operator works differently depending on what .prop looks like:

Changing properties: If there is an own data property .prop , assignment changes its value to value .

Invoking setters: If there is an own or inherited setter for .prop , assignment invokes that setter.

Creating properties: If there is no own data property .prop and no own or inherited setter for it, assignment creates a new own data property.

That is, the main purpose of assignment is making changes. That’s why it supports setters.

11.1.2 Definition

To define a property with the key propKey of an object obj , we use an operation such as the following method:

This method works differently depending on what the property looks like:

• Changing properties: If an own property with key propKey exists, defining changes its property attributes as specified by the property descriptor propDesc (if possible).
• Creating properties: Otherwise, defining creates an own property with the attributes specified by propDesc (if possible).

That is, the main purpose of definition is to create an own property (even if there is an inherited setter, which it ignores) and to change property attributes.

11.2 Assignment and definition in theory (optional)

In specification operations, property descriptors are not JavaScript objects but Records , a spec-internal data structure that has fields . The keys of fields are written in double brackets. For example, Desc.[[Configurable]] accesses the field .[[Configurable]] of Desc . These records are translated to and from JavaScript objects when interacting with the outside world.

11.2.1 Assigning to a property

The actual work of assigning to a property is handled via the following operation in the ECMAScript specification:

These are the parameters:

• O is the object that is currently being visited.
• P is the key of the property that we are assigning to.
• V is the value we are assigning.
• Receiver is the object where the assignment started.
• ownDesc is the descriptor of O[P] or null if that property doesn’t exist.

The return value is a boolean that indicates whether or not the operation succeeded. As explained later in this chapter , strict-mode assignment throws a TypeError if OrdinarySetWithOwnDescriptor() fails.

This is a high-level summary of the algorithm:

• It traverses the prototype chain of Receiver until it finds a property whose key is P . The traversal is done by calling OrdinarySetWithOwnDescriptor() recursively. During recursion, O changes and points to the object that is currently being visited, but Receiver stays the same.
• Depending on what the traversal finds, an own property is created in Receiver (where recursion started) or something else happens.

In more detail, this algorithm works as follows:

If O has a prototype parent , then we return parent.[[Set]](P, V, Receiver) . This continues our search. The method call usually ends up invoking OrdinarySetWithOwnDescriptor() recursively.

Otherwise, our search for P has failed and we set ownDesc as follows:

With this ownDesc , the next if statement will create an own property in Receiver .

• If ownDesc.[[Writable]] is false , return false . This means that any non-writable property P (own or inherited!) prevents assignment.
• The current object O and the current property descriptor ownDesc on one hand.
• The original object Receiver and the original property descriptor existingDescriptor on the other hand.
• (If we get here, then we are still at the beginning of the prototype chain – we only recurse if Receiver does not have a property P .)
• If existingDescriptor specifies an accessor, return false .
• If existingDescriptor.[[Writable]] is false , return false .
• Return Receiver.[[DefineOwnProperty]](P, { [[Value]]: V }) . This internal method performs definition, which we use to change the value of property Receiver[P] . The definition algorithm is described in the next subsection.
• (If we get here, then Receiver does not have an own property with key P .)
• Return CreateDataProperty(Receiver, P, V) . ( This operation creates an own data property in its first argument.)
• (If we get here, then ownDesc describes an accessor property that is own or inherited.)
• Let setter be ownDesc.[[Set]] .
• If setter is undefined , return false .
• Perform Call(setter, Receiver, «V») . Call() invokes the function object setter with this set to Receiver and the single parameter V (French quotes «» are used for lists in the specification).

Return true .

11.2.1.1 How do we get from an assignment to OrdinarySetWithOwnDescriptor() ?

Evaluating an assignment without destructuring involves the following steps:

• In the spec, evaluation starts in the section on the runtime semantics of AssignmentExpression . This section handles providing names for anonymous functions, destructuring, and more.
• If there is no destructuring pattern, then PutValue() is used to make the assignment.
• For property assignments, PutValue() invokes the internal method .[[Set]]() .
• For ordinary objects, .[[Set]]() calls OrdinarySet() (which calls OrdinarySetWithOwnDescriptor() ) and returns the result.

Notably, PutValue() throws a TypeError in strict mode if the result of .[[Set]]() is false .

11.2.2 Defining a property

The actual work of defining a property is handled via the following operation in the ECMAScript specification:

The parameters are:

• The object O where we want to define a property. There is a special validation-only mode where O is undefined . We are ignoring this mode here.
• The property key P of the property we want to define.
• extensible indicates if O is extensible.
• Desc is a property descriptor specifying the attributes we want the property to have.
• current contains the property descriptor of an own property O[P] if it exists. Otherwise, current is undefined .

The result of the operation is a boolean that indicates if it succeeded. Failure can have different consequences. Some callers ignore the result. Others, such as Object.defineProperty() , throw an exception if the result is false .

This is a summary of the algorithm:

If current is undefined , then property P does not currently exist and must be created.

• If extensible is false , return false indicating that the property could not be added.
• Otherwise, check Desc and create either a data property or an accessor property.

If Desc doesn’t have any fields, return true indicating that the operation succeeded (because no changes had to be made).

If current.[[Configurable]] is false :

• ( Desc is not allowed to change attributes other than value .)
• If Desc.[[Configurable]] exists, it must have the same value as current.[[Configurable]] . If not, return false .
• Same check: Desc.[[Enumerable]]

Next, we validate the property descriptor Desc : Can the attributes described by current be changed to the values specified by Desc ? If not, return false . If yes, go on.

• If the descriptor is generic (with no attributes specific to data properties or accessor properties), then validation is successful and we can move on.
• The current property must be configurable (otherwise its attributes can’t be changed as necessary). If not, false is returned.
• Change the current property from a data property to an accessor property or vice versa. When doing so, the values of .[[Configurable]] and .[[Enumerable]] are preserved, all other attributes get default values ( undefined for object-valued attributes, false for boolean-valued attributes).
• (Due to current.[[Configurable]] being false , Desc.[[Configurable]] and Desc.[[Enumerable]] were already checked previously and have the correct values.)
• If Desc.[[Writable]] exists and is true , then return false .
• If Desc.[[Value]] exists and does not have the same value as current.[[Value]] , then return false .
• There is nothing more to do. Return true indicating that the algorithm succeeded.
• (Note that normally, we can’t change any attributes of a non-configurable property other than its value. The one exception to this rule is that we can always go from writable to non-writable. This algorithm handles this exception correctly.)
• If Desc.[[Set]] exists, it must have the same value as current.[[Set]] . If not, return false .
• Same check: Desc.[[Get]]

Set the attributes of the property with key P to the values specified by Desc . Due to validation, we can be sure that all of the changes are allowed.

11.3 Definition and assignment in practice

This section describes some consequences of how property definition and assignment work.

11.3.1 Only definition allows us to create a property with arbitrary attributes

If we create an own property via assignment, it always creates properties whose attributes writable , enumerable , and configurable are all true .

Therefore, if we want to specify arbitrary attributes, we must use definition.

And while we can create getters and setters inside object literals, we can’t add them later via assignment. Here, too, we need definition.

11.3.2 The assignment operator does not change properties in prototypes

Let us consider the following setup, where obj inherits the property prop from proto .

We can’t (destructively) change proto.prop by assigning to obj.prop . Doing so creates a new own property:

The rationale for this behavior is as follows: Prototypes can have properties whose values are shared by all of their descendants. If we want to change such a property in only one descendant, we must do so non-destructively, via overriding. Then the change does not affect the other descendants.

11.3.3 Assignment calls setters, definition doesn’t

What is the difference between defining the property .prop of obj versus assigning to it?

If we define, then our intention is to either create or change an own (non-inherited) property of obj . Therefore, definition ignores the inherited setter for .prop in the following example:

If, instead, we assign to .prop , then our intention is often to change something that already exists and that change should be handled by the setter:

11.3.4 Inherited read-only properties prevent creating own properties via assignment

What happens if .prop is read-only in a prototype?

In any object that inherits the read-only .prop from proto , we can’t use assignment to create an own property with the same key – for example:

Why can’t we assign? The rationale is that overriding an inherited property by creating an own property can be seen as non-destructively changing the inherited property. Arguably, if a property is non-writable, we shouldn’t be able to do that.

However, defining .prop still works and lets us override:

Accessor properties that don’t have a setter are also considered to be read-only:

The fact that read-only properties prevent assignment earlier in the prototype chain, has been given the name override mistake :

• It was introduced in ECMAScript 5.1.
• On one hand, this behavior is consistent with how prototypal inheritance and setters work. (So, arguably, it is not a mistake.)
• On the other hand, with the behavior, deep-freezing the global object causes unwanted side-effects.
• There was an attempt to change the behavior, but that broke the library Lodash and was abandoned ( pull request on GitHub ).
• Pull request on GitHub
• Wiki page on ECMAScript.org ( archived )

11.4 Which language constructs use definition, which assignment?

In this section, we examine where the language uses definition and where it uses assignment. We detect which operation is used by tracking whether or not inherited setters are called. See §11.3.3 “Assignment calls setters, definition doesn’t” for more information.

11.4.1 The properties of an object literal are added via definition

When we create properties via an object literal, JavaScript always uses definition (and therefore never calls inherited setters):

11.4.2 The assignment operator = always uses assignment

The assignment operator = always uses assignment to create or change properties.

11.4.3 Public class fields are added via definition

Alas, even though public class fields have the same syntax as assignment, they do not use assignment to create properties, they use definition (like properties in object literals):

11.5 Further reading and sources of this chapter

Section “Prototype chains” in “JavaScript for impatient programmers”

Email by Allen Wirfs-Brock to the es-discuss mailing list : “The distinction between assignment and definition […] was not very important when all ES had was data properties and there was no way for ES code to manipulate property attributes.” [That changed with ECMAScript 5.]

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Property accessors

There are two ways of setting, altering, and accessing the properties of an object: dot notation and bracket notation .

Dot notation

As seen in some previous examples, dot notation uses a period ( . ) between an object and the property key being accessed:

You can use dot notation to access, change, or create new properties using assignment operators :

Chaining property keys using dot notation lets you access the properties of objects that are themselves properties of an object:

However, using this syntax when a key specified in the chain might not be defined can cause errors. In the following example, myMissingProp isn't a property of the myObj object, so trying to access myObj.myMissingProp results in undefined . Trying to then access a property on undefined , as if it were an object, causes an error:

To address this issue, ES2020 introduced an "optional chaining operator" ( ?. ) to safely access nested properties of objects.

Keys accessed using dot notation aren't enclosed in quotation marks like string literals are. This means you can use dot notation to access only property keys that are valid identifiers :

Because of this, it's best practice to follow identifier rules when specifying property keys whenever possible. If this isn't possible for a given key, an alternate bracket notation syntax lets you set and access string-based object keys that don't follow identifier rules.

Bracket notation

Bracket notation uses a set of brackets ( [] ) containing a value that evaluates a string (or a Symbol ) representing the property key.

This syntax is considerably more permissive, and potentially permissive enough to be confusing, because the value in brackets is evaluated as a string literal regardless of its data type. For example, here the boolean value false and the number value 10 are used to access properties associated with the string literal keys "false" and "10" :

The strength of this syntax is in its flexibility, allowing the use of dynamically-created strings to access properties. The following example uses a random number to select one of an object's three properties:

As with dot notation, you can use bracket notation to both access and create new properties, using assignment operators :

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License , and code samples are licensed under the Apache 2.0 License . For details, see the Google Developers Site Policies . Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2024-03-31 UTC.

Fix – Property assignment expected. ts(1136)

The error “error TS1136: Property assignment expected.” In typescript and Javascript occurs for various reason. In this ts tutorial, we will see why the error occurs and how to solve it in VSCODE. The error mainly occurs when you have an already existing babel project and you try to transition it to Typescript . The compiler will throw an error in reference to object destructuring codes like var user = {...this.props.userName}; and you might think the syntax of object destructuring in JS is different from typescript if the code previously work in babel. But the syntax is the same just that with TS you have to use an interface to define your state.

We have defined a state using an interface which represent the structure of how our user object is going to be like. Then we did the destructuring by assigning a value to the ‘userName’ property which already exist. And in the last line of the above code, we appended a new property to the state, but bear in mind is not part of the properties defined in the interface. So why then did we not get an error that says ‘userAge’ is not a known property of State? Well note that we did used …user to append the ‘userAge’ to whatever properties already exist in the user variable (we weren’t appending a new property to the state itself).

With the interface already defined, we can move ahead and check the properties and emit some values.

Check also, Property does not exist on type ‘{}’ in TypeScript

Also, if you try to create a simple hello world application and inside the render() method you surround the return statement with a curry-braces instead of parenthesis, you will encounter the above error. Look at below code example and how it can be solved.

There is a bug in the above code and if we run the application this is what is presented on the console.

error message: at <div , I got “Property assignment expected.ts(1136)” at className, I got 2 errors Unreachable code detected.ts(7027) ‘;’ expected.ts(1005)

Check also, Typescript Tuples: Explained (With Examples)

The Solution, is to simply surround the return statement with a parenthesis instead of curry-braces as we have done below.

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JavaScript Debugging Toolkit: Identifying and Fixing "Invalid assignment left-hand side"

This error arises when you attempt to assign a value to something that cannot be assigned to. JavaScript requires valid "left-hand sides" (targets) for assignments, which are typically variables, object properties, or array elements.

Correct Usage:

• Declared variables ( var , let , or const )
• Existing variables
• Object properties directly (without functions)
• Array elements using their numerical indices ( myArray[0] = 5 )

Incorrect Usage:

• Attempting to assign to expressions or values returned by functions
• Assigning to undeclared variables (causes ReferenceError )
• Using incorrect keywords or operators (e.g., using = for comparison instead of == or === )

Sample Code:

Precautions:

• Carefully check variable declaration (using var , let , or const ) to avoid undeclared variable errors.
• Remember that constants ( const ) cannot be reassigned after declaration.
• Use == or === for comparisons, not = for assignments.
• Be mindful of operator precedence (assignment has lower precedence than logical operators like && ).
• For object properties and array elements, ensure the object or array exists before assignment.
• ReferenceError: Occurs when trying to assign to an undeclared variable.
• TypeError: Occurs when trying to assign to a value that cannot be hold a value (e.g., modifying a constant or a returned function value).
• SyntaxError: Occurs if the code has incorrect syntax issues that prevent parsing.

Key Points:

• Understand the different assignment operators and when to use them.
• Declare variables before using them (except var , which has hoisting).
• Be mindful of object property and array element accessibility.
• Use strict equality comparison ( === ) or loose equality ( == ) instead of single assignment ( = ) for comparisons.
• Practice debugging techniques to identify and fix assignment errors.
• Consider using linters or code analysis tools to catch potential errors early.

By following these guidelines and carefully avoiding incorrect assignment scenarios, you can write clearer, more robust JavaScript code.

The JavaScript exception "invalid assignment left-hand side" occurs when there was an unexpected assignment somewhere. It may be triggered when a single = sign was used instead of == or === .

SyntaxError or ReferenceError , depending on the syntax.

What went wrong?

There was an unexpected assignment somewhere. This might be due to a mismatch of an assignment operator and an equality operator , for example. While a single = sign assigns a value to a variable, the == or === operators compare a value.

Typical invalid assignments

In the if statement, you want to use an equality operator ( === ), and for the string concatenation, the plus ( + ) operator is needed.

Assignments producing ReferenceErrors

Invalid assignments don't always produce syntax errors. Sometimes the syntax is almost correct, but at runtime, the left hand side expression evaluates to a value instead of a reference , so the assignment is still invalid. Such errors occur later in execution, when the statement is actually executed.

Function calls, new calls, super() , and this are all values instead of references. If you want to use them on the left hand side, the assignment target needs to be a property of their produced values instead.

Note: In Firefox and Safari, the first example produces a ReferenceError in non-strict mode, and a SyntaxError in strict mode . Chrome throws a runtime ReferenceError for both strict and non-strict modes.

Using optional chaining as assignment target

Optional chaining is not a valid target of assignment.

Instead, you have to first guard the nullish case.

• Assignment operators
• Equality operators

© 2005–2023 MDN contributors. Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Invalid_assignment_left-hand_side

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JS Tutorial

Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript errors, throw, and try...catch...finally.

The try statement defines a code block to run (to try).

The catch statement defines a code block to handle any error.

The finally statement defines a code block to run regardless of the result.

The throw statement defines a custom error.

Errors Will Happen!

When executing JavaScript code, different errors can occur.

Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things.

In this example we misspelled "alert" as "adddlert" to deliberately produce an error:

JavaScript catches adddlert as an error, and executes the catch code to handle it.

JavaScript try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs:

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JavaScript Throws Errors

When an error occurs, JavaScript will normally stop and generate an error message.

The technical term for this is: JavaScript will throw an exception (throw an error) .

JavaScript will actually create an Error object with two properties: name and message .

The throw Statement

The throw statement allows you to create a custom error.

Technically you can throw an exception (throw an error) .

The exception can be a JavaScript String , a Number , a Boolean or an Object :

If you use throw together with try and catch , you can control program flow and generate custom error messages.

Input Validation Example

This example examines input. If the value is wrong, an exception (err) is thrown.

The exception (err) is caught by the catch statement and a custom error message is displayed:

HTML Validation

The code above is just an example.

Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes:

You can read more about forms validation in a later chapter of this tutorial.

The finally Statement

The finally statement lets you execute code, after try and catch, regardless of the result:

The Error Object

JavaScript has a built in error object that provides error information when an error occurs.

The error object provides two useful properties: name and message.

Error Object Properties

Error name values.

Six different values can be returned by the error name property:

The six different values are described below.

An EvalError indicates an error in the eval() function.

Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.

Range Error

A RangeError is thrown if you use a number that is outside the range of legal values.

For example: You cannot set the number of significant digits of a number to 500.

Reference Error

A ReferenceError is thrown if you use (reference) a variable that has not been declared:

Syntax Error

A SyntaxError is thrown if you try to evaluate code with a syntax error.

A TypeError is thrown if an operand or argument is incompatible with the type expected by an operator or function.

URI (Uniform Resource Identifier) Error

A URIError is thrown if you use illegal characters in a URI function:

Non-Standard Error Object Properties

Mozilla and Microsoft define some non-standard error object properties:

fileName (Mozilla) lineNumber (Mozilla) columnNumber (Mozilla) stack (Mozilla) description (Microsoft) number (Microsoft)

Do not use these properties in public web sites. They will not work in all browsers.

Complete Error Reference

For a complete reference of the Error object, go to our Complete JavaScript Error Reference .

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Property flags and descriptors

As we know, objects can store properties.

Until now, a property was a simple “key-value” pair to us. But an object property is actually a more flexible and powerful thing.

In this chapter we’ll study additional configuration options, and in the next we’ll see how to invisibly turn them into getter/setter functions.

Property flags

Object properties, besides a value , have three special attributes (so-called “flags”):

• writable – if true , the value can be changed, otherwise it’s read-only.
• enumerable – if true , then listed in loops, otherwise not listed.
• configurable – if true , the property can be deleted and these attributes can be modified, otherwise not.

We didn’t see them yet, because generally they do not show up. When we create a property “the usual way”, all of them are true . But we also can change them anytime.

First, let’s see how to get those flags.

The method Object.getOwnPropertyDescriptor allows to query the full information about a property.

The syntax is:

The returned value is a so-called “property descriptor” object: it contains the value and all the flags.

For instance:

To change the flags, we can use Object.defineProperty .

If the property exists, defineProperty updates its flags. Otherwise, it creates the property with the given value and flags; in that case, if a flag is not supplied, it is assumed false .

For instance, here a property name is created with all falsy flags:

Compare it with “normally created” user.name above: now all flags are falsy. If that’s not what we want then we’d better set them to true in descriptor .

Now let’s see effects of the flags by example.

Non-writable

Let’s make user.name non-writable (can’t be reassigned) by changing writable flag:

Now no one can change the name of our user, unless they apply their own defineProperty to override ours.

In non-strict mode, no errors occur when writing to non-writable properties and such. But the operation still won’t succeed. Flag-violating actions are just silently ignored in non-strict.

Here’s the same example, but the property is created from scratch:

Non-enumerable

Now let’s add a custom toString to user .

Normally, a built-in toString for objects is non-enumerable, it does not show up in for..in . But if we add a toString of our own, then by default it shows up in for..in , like this:

If we don’t like it, then we can set enumerable:false . Then it won’t appear in a for..in loop, just like the built-in one:

Non-enumerable properties are also excluded from Object.keys :

Non-configurable

The non-configurable flag ( configurable:false ) is sometimes preset for built-in objects and properties.

A non-configurable property can’t be deleted, its attributes can’t be modified.

For instance, Math.PI is non-writable, non-enumerable and non-configurable:

So, a programmer is unable to change the value of Math.PI or overwrite it.

We also can’t change Math.PI to be writable again:

There’s absolutely nothing we can do with Math.PI .

Making a property non-configurable is a one-way road. We cannot change it back with defineProperty .

Please note: configurable: false prevents changes of property flags and its deletion, while allowing to change its value.

Here user.name is non-configurable, but we can still change it (as it’s writable):

And here we make user.name a “forever sealed” constant, just like the built-in Math.PI :

There’s a minor exception about changing flags.

We can change writable: true to false for a non-configurable property, thus preventing its value modification (to add another layer of protection). Not the other way around though.

Object.defineProperties

There’s a method Object.defineProperties(obj, descriptors) that allows to define many properties at once.

So, we can set many properties at once.

Object.getOwnPropertyDescriptors

To get all property descriptors at once, we can use the method Object.getOwnPropertyDescriptors(obj) .

Together with Object.defineProperties it can be used as a “flags-aware” way of cloning an object:

Normally when we clone an object, we use an assignment to copy properties, like this:

…But that does not copy flags. So if we want a “better” clone then Object.defineProperties is preferred.

Another difference is that for..in ignores symbolic and non-enumerable properties, but Object.getOwnPropertyDescriptors returns all property descriptors including symbolic and non-enumerable ones.

Sealing an object globally

Property descriptors work at the level of individual properties.

There are also methods that limit access to the whole object:

And also there are tests for them:

These methods are rarely used in practice.

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Home » JavaScript Tutorial » JavaScript Object Destructuring

JavaScript Object Destructuring

Summary : in this tutorial, you’ll learn about JavaScript object destructuring which assigns properties of an object to individual variables.

If you want to learn how to destructure an array , you can check out the array destructuring tutorial .

Introduction to the JavaScript object destructuring assignment

Suppose you have a person object with two properties: firstName and lastName .

Before ES6, when you want to assign properties of the person object to variables, you typically do it like this:

ES6 introduces the object destructuring syntax that provides an alternative way to assign properties of an object to variables:

In this example, the firstName and lastName properties are assigned to the fName and lName variables respectively.

In this syntax:

The identifier before the colon ( : ) is the property of the object and the identifier after the colon is the variable.

Notice that the property name is always on the left whether it’s an object literal or object destructuring syntax.

If the variables have the same names as the properties of the object, you can make the code more concise as follows:

In this example, we declared two variables firstName and lastName , and assigned the properties of the person object to the variables in the same statement.

It’s possible to separate the declaration and assignment. However, you must surround the variables in parentheses:

If you don’t use the parentheses, the JavaScript engine will interpret the left-hand side as a block and throw a syntax error.

When you assign a property that does not exist to a variable using the object destructuring, the variable is set to undefined . For example:

In this example, the middleName property doesn’t exist in the person object, therefore, the middleName variable is undefined .

Setting default values

You can assign a default value to the variable when the property of an object doesn’t exist. For example:

In this example, we assign an empty string to the middleName variable when the person object doesn’t have the middleName property.

Also, we assign the currentAge property to the age variable with the default value of 18.

However, when the person object does have the middleName property, the assignment works as usual:

Destructuring a null object

A function may return an object or null in some situations. For example:

And you use the object destructuring assignment:

The code will throw a TypeError :

To avoid this, you can use the OR operator ( || ) to fallback the null object to an empty object:

Now, no error will occur. And the firstName and lastName will be undefined .

Nested object destructuring

Assuming that you have an employee object which has a name object as the property:

The following statement destructures the properties of the nested name object into individual variables:

It’s possible to do multiple assignment of a property to multiple variables:

Destructuring function arguments

Suppose you have a function that displays the person object:

It’s possible to destructure the object argument passed into the function like this:

It looks less verbose especially when you use many properties of the argument object. This technique is often used in React.

• Object destructuring assigns the properties of an object to variables with the same names by default.

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TypeScript 对象解构导致“Property assignment expected.”错误

在本文中，我们将介绍TypeScript中对象解构导致的常见错误”Property assignment expected.”，并提供解决方案和示例。

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对象解构是一种从对象中提取数据并将其赋值给变量的方法。它允许我们通过简洁的语法访问和使用对象中的属性和方法。在TypeScript中，我们可以使用解构来从对象中获取属性并将其赋给变量。

“Property assignment expected.”错误

在使用对象解构时，有时我们可能会遇到错误消息”Property assignment expected.”。这个错误通常是由以下几种情况引起的。

1. 解构引用的属性不存在

如果我们尝试解构一个对象中不存在的属性，就会导致”Property assignment expected.”错误。

在上面的示例中，我们尝试解构 person 对象的 age 属性，但是 person 对象中并没有 age 属性。因此，这将导致错误的发生。为了避免这个错误，我们应该确保解构引用的属性存在于对象中。

2. 解构引用的属性未定义

当我们尝试解构一个未定义的属性时，也会导致”Property assignment expected.”错误。

在上面的示例中，我们声明了一个类型为 { name: string; age: number } 的变量 person ，但是我们没有为其赋值。因此，当我们尝试解构 person 对象的 name 和 age 属性时，它们都是未定义的。为了避免这个错误，我们应该确保在解构之前给对象的属性赋值。

当我们在解构语句中使用了错误的语法时，也会导致”Property assignment expected.”错误。

在上面的示例中，我们在解构语句中错误地省略了 age 属性之后的变量名。这将导致解构语法错误，从而导致”Property assignment expected.”错误。为了避免这个错误，我们应该确保在解构语句中使用正确的语法。

要解决”Property assignment expected.”错误，我们可以采取以下一些方法。

1. 确保解构引用的属性存在于对象中

在使用对象解构之前，我们应该确保解构引用的属性存在于对象中。

上面的示例中，我们尝试解构 person 对象的 name 和 age 属性，但是 person 对象中只有 name 属性。要修复这个错误，我们可以在解构之前添加一个判断，以确保属性存在。

2. 给解构引用的属性赋予默认值

当我们解构一个对象中可能不存在的属性时，可以给解构引用的属性赋予默认值。

上面的示例中，我们给解构引用的 age 属性赋予了默认值30。如果 person 对象中不存在 age 属性，那么解构后的 age 变量将等于默认值30。

3. 使用正确的解构语法

为了避免解构语法错误，我们应该确保使用正确的语法。

上面的示例中，我们使用正确的解构语法来同时解构 person 对象的 name 和 age 属性。

当使用TypeScript中的对象解构时，我们可能会遇到”Property assignment expected.”错误。这个错误通常是由解构引用的属性不存在、解构引用的属性未定义或解构语法错误引起的。要解决这个错误，我们可以确保解构引用的属性存在于对象中、给解构引用的属性赋予默认值或使用正确的解构语法。希望本文提供的解决方案和示例对您有帮助！

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The throw statement throws a user-defined exception. Execution of the current function will stop (the statements after throw won't be executed), and control will be passed to the first catch block in the call stack. If no catch block exists among caller functions, the program will terminate.

The expression to throw.

Description

The throw statement is valid in all contexts where statements can be used. Its execution generates an exception that penetrates through the call stack. For more information on error bubbling and handling, see Control flow and error handling .

The throw keyword can be followed by any kind of expression, for example:

In practice, the exception you throw should always be an Error object or an instance of an Error subclass, such as RangeError . This is because code that catches the error may expect certain properties, such as message , to be present on the caught value. For example, web APIs typically throw DOMException instances, which inherit from Error.prototype .

Automatic semicolon insertion

The syntax forbids line terminators between the throw keyword and the expression to be thrown.

The code above is transformed by automatic semicolon insertion (ASI) into:

This is invalid code, because unlike return , throw must be followed by an expression.

To avoid this problem (to prevent ASI), you could use parentheses:

Throwing a user-defined error

This example defines a function that throws a TypeError if the input is not of the expected type.

Throwing an existing object

This example calls a callback-based async function, and throws an error if the callback receives an error.

Errors thrown this way are not catchable by the caller and will cause the program to crash unless (a) the readFile function itself catches the error, or (b) the program is running in a context that catches top-level errors. You can handle errors more naturally by using the Promise() constructor.

Specifications

Browser compatibility.

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• try...catch