Category: JavaScript

Sometimes, we may run into the ‘SyntaxError: "x" is a reserved identifier’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: "x" is a reserved identifier’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: "x" is a reserved identifier’ When Developing JavaScript Apps

To fix the ‘SyntaxError: "x" is a reserved identifier’ when we’re developing JavaScript apps, we should make sure we aren’t trying to use reserved keywords for variable names.

In Edge, the error message for this error is SyntaxError: The use of a future reserved word for an identifier is invalid.

In Firefox, the error message for this error is SyntaxError: "x" is a reserved identifier.

And in Chrome, the error message for this error is SyntaxError: Unexpected reserved word.

enum is a reserved keyword in strict or sloppy mode.

The following keywords are reserved keywords in strict mode only:

• implements
• interface
• let
• package
• private
• protected
• public
• static

For instance, we shouldn’t write code like"

const enum = { RED: 0, GREEN: 1, BLUE: 2 };

Also, we shouldn’t write coee like:

"use strict";
const package = ["potatoes", "apples", "fries"];

Instead, we should rename them so their names aren’t in the reserved keywords list:

const colorEnum = { RED: 0, GREEN: 1, BLUE: 2 };
const list = ["potatoes", "rice", "fries"];

Conclusion

To fix the ‘SyntaxError: "x" is a reserved identifier’ when we’re developing JavaScript apps, we should make sure we aren’t trying to use reserved keywords for variable names.

In Edge, the error message for this error is SyntaxError: The use of a future reserved word for an identifier is invalid.

In Firefox, the error message for this error is SyntaxError: "x" is a reserved identifier.

And in Chrome, the error message for this error is SyntaxError: Unexpected reserved word.

Sometimes, we may run into the ‘SyntaxError: "use strict" not allowed in function with non-simple parameters’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: "use strict" not allowed in function with non-simple parameters’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: "use strict" not allowed in function with non-simple parameters’ When Developing JavaScript Apps

To fix the ‘SyntaxError: "use strict" not allowed in function with non-simple parameters’ when we’re developing JavaScript apps, we should make sure we don’t add the 'use strict' directive inside a function that uses default parameters, rest parameters, or destructuring parameters.

In Edge, the error message for this error is Cannot apply strict mode on functions with non-simple parameter list .

In Firefox, the error message for this error is SyntaxError: "use strict" not allowed in function with default parameter, SyntaxError: "use strict" not allowed in function with rest parameter, and SyntaxError: "use strict" not allowed in function with destructuring parameter.

And in Chrome, the error message for this error is SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list.

For instance, we shouldn’t write code like:

const sum = (a = 1, b = 2) => {
  'use strict';
  return a + b;
}

Instead, we should put 'use strict' before the function:

"use strict";
const sum = (a = 1, b = 2) => {
  return a + b;
};

Conclusion

To fix the ‘SyntaxError: "use strict" not allowed in function with non-simple parameters’ when we’re developing JavaScript apps, we should make sure we don’t add the 'use strict' directive inside a function that uses default parameters, rest parameters, or destructuring parameters.

In Edge, the error message for this error is Cannot apply strict mode on functions with non-simple parameter list .

In Firefox, the error message for this error is SyntaxError: "use strict" not allowed in function with default parameter, SyntaxError: "use strict" not allowed in function with rest parameter, and SyntaxError: "use strict" not allowed in function with destructuring parameter.

And in Chrome, the error message for this error is SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list.

Sometimes, we may run into the ‘SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated’ When Developing JavaScript Apps

To fix the ‘SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated’ when we’re developing JavaScript apps, we should make sure that we don’t use 0-prefixed numbers to add octal numbers in our JavaScript code.

For instance, we shouldn’t write code like:

03;

in our code.

Instead, we write:

0o3;

to add an octal number.

We should also use hex escape sequences instead of octal escape sequences:

'\xA9';

Conclusion

To fix the ‘SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated’ when we’re developing JavaScript apps, we should make sure that we don’t use 0-prefixed numbers to add octal numbers in our JavaScript code.

We should also use hex escape sequences instead of octal escape sequences.

Sometimes, we may run into the ‘ReferenceError: invalid assignment left-hand side’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘ReferenceError: invalid assignment left-hand side’ when we’re developing JavaScript apps.

Fix the ‘ReferenceError: invalid assignment left-hand side’ When Developing JavaScript Apps

To fix the ‘ReferenceError: invalid assignment left-hand side’ when we’re developing JavaScript apps, we should make sure that we’re accidentally using the = operator for doing comparisons.

Also, we should make sure our assignment statements have variables on the left side to assign to.

For instance, we’ll get this error is we write something like:

if (Math.PI = 3 || Math.PI = 4) {
  console.log('error');
}

Also, we should make sure that we have a variable on the left side of the assignment statement.

Therefore, we shouldn’t write statements like:

let  str = 'Hello, '
+= 'is it what '
+= 'you\'re looking for?';

To fix the first piece of code, we write:

if (Math.PI === 3 || Math.PI === 4) {
  console.log('error');
}

And to fix the 2nd piece of code, we write:

let str = "Hello, ";
str += "is it what ";
str += "you're looking for?";

Conclusion

To fix the ‘ReferenceError: invalid assignment left-hand side’ when we’re developing JavaScript apps, we should make sure that we’re accidentally using the = operator for doing comparisons.

Also, we should make sure our assignment statements have variables on the left side to assign to.

Sometimes, we may run into the ‘ReferenceError: deprecated caller or arguments usage’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘ReferenceError: deprecated caller or arguments usage’ when we’re developing JavaScript apps.

Fix the ‘ReferenceError: deprecated caller or arguments usage’ When Developing JavaScript Apps

To fix the ‘ReferenceError: deprecated caller or arguments usage’ when we’re developing JavaScript apps, we should make sure we aren’t trying to access the Function.caller or arguments.callee.callers properties in our JavaScript code.

They’re both deprecated properties that will be removed in the future.

For instance, we should write code like:

'use strict';

function myFunc() {
  if (myFunc.caller == null) {
    return "The function was called from the top";
  } else {
    return myFunc.caller;
  }
}

myFunc();

Also, we shouldn’t access the Function.arguments property in our code:

"use strict";

function f(n) {
  g(n - 1);
}

function g(n) {
  console.log("before: " + g.arguments[0]);
  if (n > 0) {
    f(n);
  }
  console.log("after: " + g.arguments[0]);
}

f(2);

We should get function arguments from the signature of the function directly or with the rest operator:

"use strict";

function f(n) {
  g(n - 1);
}

function g(...args) {
  console.log("before: " + args[0]);
  if (n > 0) {
    f(n);
  }
  console.log("after: " + args[0]);
}

f(2);

args is an array with all the arguments g is called with.

Conclusion

To fix the ‘ReferenceError: deprecated caller or arguments usage’ when we’re developing JavaScript apps, we should make sure we aren’t trying to access the Function.caller or arguments.callee.callers properties in our JavaScript code.

They’re both deprecated properties that will be removed in the future.