Author: John Au-Yeung

Sometimes, we may run into the ‘SyntaxError: function statement requires a name’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: function statement requires a name’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: function statement requires a name’ When Developing JavaScript Apps

To fix the ‘SyntaxError: function statement requires a name’ when we’re developing JavaScript apps, we should make sure that our function declarations has a function name in it.

On Edge, the error message for this error is Syntax Error: Expected identifier.

On Firefox, the error message for this error is SyntaxError: function statement requires a name.

And on Chrome, the error message for this error is SyntaxError: Unexpected token.

Therefore, the following code will throw the error:

function () {
  return 'Hello world';
}

Instead, we should write:

function greet () {
  return 'Hello world';
}

or:

const greet = function() {
  return 'Hello world';
};

We can also use an anonymous function in an IIFE:

(function () {
  return 'Hello world';
})()

Also, we should make sure that our callbacks don’t have syntax errors in them.

So instead of writing:

promise.then(
  function() {
    console.log("success");
  });
  function() {
    console.log("error");
}

which has misplaced parentheses, we write:

promise.then(
  function() {
    console.log("success");
  },
  function() {
    console.log("error");
  }
);

Conclusion

To fix the ‘SyntaxError: function statement requires a name’ when we’re developing JavaScript apps, we should make sure that our function declarations has a function name in it.

On Edge, the error message for this error is Syntax Error: Expected identifier.

On Firefox, the error message for this error is SyntaxError: function statement requires a name.

And on Chrome, the error message for this error is SyntaxError: Unexpected token.

Sometimes, we may run into the ‘SyntaxError: for-in loop head declarations may not have initializers’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: for-in loop head declarations may not have initializers’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: for-in loop head declarations may not have initializers’ When Developing JavaScript Apps

To fix the ‘SyntaxError: for-in loop head declarations may not have initializers’ when we’re developing JavaScript apps, we should make sure that we don’t have variable assignment statements in the head of the for-in loop.

On Edge, the error message for this error is SyntaxError: for-in loop head declarations cannot have an initializer.

On Firefox, the error message for this error is SyntaxError: for-in loop head declarations may not have initializers.

And on Chrome, the error message for this error is SyntaxError: for-in loop variable declaration may not have an initializer..

For instance, we can’t write:

"use strict";

const obj = {
  a: 1,
  b: 2,
  c: 3
};

for (const i = 0 in obj) {
  console.log(obj[i]);
}

since we’re assigning a value in the head of the for-in loop, which is invalid syntax.

Instead, we write:

"use strict";

const obj = { a: 1, b: 2, c: 3 };

for (let i in obj) {
  i = 0;
  console.log(obj[i]);
}

which is valid syntax.

We can also use a regular for loop if we want to assign variables in the head of the loop.

Conclusion

To fix the ‘SyntaxError: for-in loop head declarations may not have initializers’ when we’re developing JavaScript apps, we should make sure that we don’t have variable assignment statements in the head of the for-in loop.

We can also use a regular for loop if we want to assign variables in the head of the loop.

On Edge, the error message for this error is SyntaxError: for-in loop head declarations cannot have an initializer.

On Firefox, the error message for this error is SyntaxError: for-in loop head declarations may not have initializers.

And on Chrome, the error message for this error is SyntaxError: for-in loop variable declaration may not have an initializer..

Sometimes, we may run into the ‘SyntaxError: applying the "delete" operator to an unqualified name is deprecated’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: applying the "delete" operator to an unqualified name is deprecated’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: applying the "delete" operator to an unqualified name is deprecated’ When Developing JavaScript Apps

To fix the ‘SyntaxError: applying the "delete" operator to an unqualified name is deprecated’ when we’re developing JavaScript apps, we should make sure that we’re using the delete operator on object properties rather than variables.

The error message for this error is SyntaxError: Calling delete on expression not allowed in strict mode on Edge.

The error message for this error is SyntaxError: applying the 'delete' operator to an unqualified name is deprecated on Firefox.

And in Chrome, the error message for this error is SyntaxError: Delete of an unqualified identifier in strict mode.

This error is only thrown in strict mode.

For instance, the error will be thrown if we write:

'use strict';

let x;

// ...

delete x;

Instead, we should set variable values to null or undefined to make the JavaScript engine garbage collect the variable.

So we can write:

'use strict';

let x;

// ...

x = undefined;

or:

'use strict';

let x;

// ...

x = null;

Conclusion

To fix the ‘SyntaxError: applying the "delete" operator to an unqualified name is deprecated’ when we’re developing JavaScript apps, we should make sure that we’re using the delete operator on object properties rather than variables.

The error message for this error is SyntaxError: Calling delete on expression not allowed in strict mode on Edge.

The error message for this error is SyntaxError: applying the 'delete' operator to an unqualified name is deprecated on Firefox.

And in Chrome, the error message for this error is SyntaxError: Delete of an unqualified identifier in strict mode.

This error is only thrown in strict mode.

Sometimes, we may run into the ‘SyntaxError: a declaration in the head of a for-of loop can’t have an initializer’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: a declaration in the head of a for-of loop can’t have an initializer’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: a declaration in the head of a for-of loop can’t have an initializer’ When Developing JavaScript Apps

To fix the ‘SyntaxError: a declaration in the head of a for-of loop can’t have an initializer’ when we’re developing JavaScript apps, we should make sure that we don’t have any assignment statements in the parentheses for the for-of loop.

For instance, instead of writing code like:

const arr = [10, 20, 30];

for (let value = 50 of arr ) {
  console.log(value);
}

where we have an invalid assignment statement in the head of the for-of loop, we write:

let value = 0;

const arr = [10, 20, 30];

for (const value of arr) {
  value += 50;
  console.log(value);
}

where we assign a value to the value variable inside the for-of loop body, which is valid.

If we want to assign a value in the head of the loop, we can use the for loop.

Conclusion

To fix the ‘SyntaxError: a declaration in the head of a for-of loop can’t have an initializer’ when we’re developing JavaScript apps, we should make sure that we don’t have any assignment statements in the parentheses for the for-of loop.

Sometimes, we may run into the ‘SyntaxError: Unexpected token’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘SyntaxError: Unexpected token’ when we’re developing JavaScript apps.

Fix the ‘SyntaxError: Unexpected token’ When Developing JavaScript Apps

To fix the ‘SyntaxError: Unexpected token’ when we’re developing JavaScript apps, we should make sure we’re writing JavaScript code that’s syntactically valid.

Different unexpected token errors that may be thrown include:

SyntaxError: expected expression, got "x"
SyntaxError: expected property name, got "x"
SyntaxError: expected target, got "x"
SyntaxError: expected rest argument name, got "x"
SyntaxError: expected closing parenthesis, got "x"
SyntaxError: expected '=>' after argument list, got "x"

For instance, we should make sure we aren’t adding trailing commas when we’re chaining expressions:

for (let i = 0; i < 5,; ++i) {
  console.log(i);
}

We have:

i < 5,;

which has a comma before the semicolon. This is invalid syntax.

Instead, we should write:

for (let i = 0; i < 5; ++i) {
  console.log(i);
}

which removed the extra comma.

We should also make sure that we have corresponding closing parentheses for any opening parentheses.

For example, instead of writing:

function round(n, upperBound, lowerBound){
  if(n > upperBound) || (n < lowerBound){
    throw 'Number ' + String(n) + ' is more than ' + String(upperBound) + ' or less than ' + String(lowerBound);
  }
} 

We write:

function round(n, upperBound, lowerBound) {
  if (n > upperBound || n < lowerBound) {
    throw (
      "Number " +
      String(n) +
      " is more than " +
      String(upperBound) +
      " or less than " +
      String(lowerBound)
    );
  }
}

which removes the extra parentheses from if boolean expression.

Conclusion

To fix the ‘SyntaxError: Unexpected token’ when we’re developing JavaScript apps, we should make sure we’re writing JavaScript code that’s syntactically valid.