ESLint error Unary operator '++' used no-plusplus [Solved]

# Table of Contents

1. ESLint error Unary operator '++' used no-plusplus
2. Disabling the ESLint no-plusplus rule globally
3. Disabling the ESLint no-plusplus rule for a single line
4. Disabling the ESLint no-plusplus rule for an entire file

# ESLint error Unary operator '++' used no-plusplus [Solved]

The ESLint error "Unary operator '++' used no-plusplus" is raised when you use the unary ++ or -- operators in your code.

To solve the error, use a reassignment instead, e.g. i += 1.

Here are 3 examples of how the error occurs.

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let i = 0;

// ⛔️ Unary operator '++' used.eslint no-plusplus
i++;

// ⛔️ Unary operator '--' used.eslint no-plusplus
i--;

// ⛔️ Unary operator '++' used. eslint no-plusplus
for (
  let index = 0;
  index < ['bobby', 'hadz', 'com'].length;
  index++
) {
  console.log(index);
}

ESLint forbids the use of the unary ++ and -- operators because they are subject to automatic semicolon insertion and differences in whitespace could change the semantics of the code.

Instead, you should use += 1 or -= 1 to reassign the variable.

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let i = 0;

i += 1;
console.log(i); // 1

i -= 1;
console.log(i); // 0

for (
  let index = 0;
  index < ['bobby', 'hadz', 'com'].length;
  index += 1
) {
  console.log(index); // 0, 1, 2
}

The += operator reassigns the variable to its current value plus 1.

Conversely, the -= operator reassigns the variable to its current value minus 1.

Here is an example of why using the unary ++ and -- operators could cause issues.

Everything in the following code sample works as expected.

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let a = 1;

let b = 2;

a++
b

// a = 2, b = 2

However, in the following code sample, the b variable gets incremented, not a.

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let a = 1;

let b = 2;

a
++
b

// a = 1, b = 3

# Disabling the ESLint no-plusplus rule globally

If you want to disable the no-plusplus rule globally, you have to edit your .eslintrc.js file.

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module.exports = {
    rules: {
        'no-plusplus': 'off',
    },
};

You could also only disable the rule in for loops.

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module.exports = {
    rules: {
      'no-plusplus': ['error', {allowForLoopAfterthoughts: true}],
    },
};

If you use a .eslintrc or .eslintrc.json file, make sure to double-quote the string properties and values.

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{
  "rules": {
    "no-plusplus": "off"
  }
}

Note that there shouldn't be a trailing comma after the last key-value pair when using JSON.

If you only want to disable the rule in for loops in a JSON config file, use the following line instead.

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{
  "rules": {
    "no-plusplus": ["error", { "allowForLoopAfterthoughts": true }]
  }
}

# Disabling the ESLint no-plusplus rule for a single line

If you want to disable the no-plusplus rule for a single line, use the following comment.

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let i = 0;

// eslint-disable-next-line no-plusplus
i++;

Note that the comment has to be added above the line on which the error occurs.

The comment disables the rule for a single line.

# Disabling the ESLint no-plusplus rule for an entire file

If you want to disable the rule for an entire file, add the following comment to the top of your file.

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/* eslint-disable no-plusplus */

let i = 0;

i++;

i--;

for (let index = 0; index < ['bobby', 'hadz', 'com'].length; index++) {
  console.log(index);
}

Make sure to add the comment at the top of your file or at least above any code that uses the unary ++ and -- operators.

The same approach can be used to disable the no-plusplus rule for a block of code.

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/* eslint-disable no-plusplus */

let i = 0;

i++;

i--;

/* eslint-enable no-plusplus */

// ⛔️ Unary operator '++' used. eslint no-plusplus
for (let index = 0; index < ['bobby', 'hadz', 'com'].length; index++) {
  console.log(index);
}

The first comment in the example disables the rule and the second comment enables it.

Using the unary ++ operator in the for loop raises the error because the for loop comes after the ESLint comment that enables the no-plusplus rule.

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