JavaScript error report Cannot set properties of undefined (setting ‘1’) solution

Preface

To find out the cause and solution of the error, you need to understand JavaScriptVariable declaration, scope, asynchronous operations and array mechanismsand other core concepts.

1. Object not initialized correctly

reason

In JavaScript, ifTrying to access or set an undefined or null object's properties will throw an error,becauseundefinedandnullNone of them are objects, and properties cannot be set.

let obj;
obj[1] = 'value'; // The obj here is undefined

The underlying principle

**In JavaScript, the default value of undeclared or uninitialized variables isundefined. **When givenundefinedWhen assigning attribute values, the JavaScript engine will throw a TypeError.

Solution

Initializing an object is the most direct solution to ensure that the variable has been correctly assigned to an object before use.

let obj = {};
obj[1] = 'value'; // Work normally

2. Array index is out of range

reason

In JavaScript, an array is an object of special type, using numbers as an index. ifTrying to access an index that is out of scope will not throw an error, it will return undefined.

let arr = [];
arr[1] = 'value'; // There are no errors here, the array will automatically expand the length(arr); // Output: [empty, "value"]

However, if you want to try to access an undefined object property, it will still cause a "Cannot set properties of undefined" error, and the problem usually occurs in complex object structures. For example:

let obj = {};
 = undefined;
[1] = 'value'; // An error will be thrown here because it is undefined

The underlying principle

In JavaScript, arrays are dynamic. When accessing an index that exceeds the current range, the array will automatically expand the length and fill it.undefined. However, if you try to access the properties of an undefined object, a TypeError will be generated.

Solution

Make sure that all intermediate objects are correctly initialized before accessing nested object properties.

let obj = {};
if (!) {
   = [];
}
[1] = 'value'; // Work normally(); // Output: [empty, "value"]

Example

How to avoid this error:

let obj = {};
function initializeSubObject() {
  if ( === undefined) {
     = [];
  }
  [1] = 'value';
}

initializeSubObject();
(); // Output: [empty, "value"]

The "Cannot set properties of undefined" error can be effectively avoided by ensuring that the nested objects are properly initialized before use.

3. Object not defined in asynchronous operation

reason

In asynchronous operations, an undefined error may result in an object that is not initialized correctly or is modified unexpectedly.

let obj;
setTimeout(() => {
  obj[1] = 'value'; // If obj is not defined in asynchronous operation}, 1000);

The underlying principle

JavaScript is single-threaded, but can handle asynchronous operations through event loop mechanisms.In an asynchronous operation, an undefined error will result in an undefined error if the object is not initialized or has been deleted before the asynchronous callback is executed.

Solution

useasync/awaitOr other asynchronous control means to ensure that the object has been initialized before asynchronous operation.

let obj;
async function setProperty() {
  obj = {};
  await new Promise(resolve => setTimeout(resolve, 1000));
  obj[1] = 'value'; // Make sure obj is defined}
setProperty();

4. Use default parameters to avoid errors

In the above error cause we found that it was an error caused by undefined, so we can use default parameters when defining the function to prevent undefined objects.

function setProperty(obj = {}) {
  obj[1] = 'value';
}
setProperty(); // Work normally

JavaScript allows the definition of default values ​​in function parameters. If the parameter is not passed or undefined when calling the function, the parameter will be assigned the default value. This avoids the hassle of handling undefined variables inside the function.

Summarize

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