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restrict-plus-operands

Require both operands of addition to be the same type and be bigint, number, or string.

🧠

Extending "plugin:@typescript-eslint/recommended-requiring-type-checking" in an ESLint configuration enables this rule.

💭

This rule requires type information to run.

TypeScript allows + adding together two values of any type(s). However, adding values that are not the same type and/or are not the same primitive type is often a sign of programmer error.

This rule reports when a + operation combines two values of different types, or a type that is not bigint, number, or string.

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/restrict-plus-operands": "error"
  }
};

Examples​

var foo = '5.5' + 5;
var foo = 1n + 1;

Options​

This rule accepts an options object with the following properties:

interface Options {
  /**
   * Whether to check compound assignments such as `+=`.
   */
  checkCompoundAssignments?: boolean;
  /**
   * Whether to allow `any` typed values.
   */
  allowAny?: boolean;
}

const defaultOptions: Options = [
  { checkCompoundAssignments: false, allowAny: false },
];

checkCompoundAssignments​

Examples of code for this rule with { checkCompoundAssignments: true }:

/*eslint @typescript-eslint/restrict-plus-operands: ["error", { "checkCompoundAssignments": true }]*/

let foo: string | undefined;
foo += 'some data';

let bar: string = '';
bar += 0;

allowAny​

Examples of code for this rule with { allowAny: true }:

var fn = (a: any, b: boolean) => a + b;
var fn = (a: any, b: []) => a + b;
var fn = (a: any, b: {}) => a + b;

When Not To Use It​

If you don't mind "[object Object]" in your strings, then you will not need this rule.

Further Reading​

Resources​