eslint/docs/src/rules/no-useless-concat.md

   1 ---
   2 title: no-useless-concat
   3 layout: doc
   4 rule_type: suggestion
   5 ---
   6
   7
   8 It's unnecessary to concatenate two strings together, such as:
   9
  10 ```js
  11 var foo = "a" + "b";
  12 ```
  13
  14 This code is likely the result of refactoring where a variable was removed from the concatenation (such as `"a" + b + "b"`). In such a case, the concatenation isn't important and the code can be rewritten as:
  15
  16 ```js
  17 var foo = "ab";
  18 ```
  19
  20 ## Rule Details
  21
  22 This rule aims to flag the concatenation of 2 literals when they could be combined into a single literal. Literals can be strings or template literals.
  23
  24 Examples of **incorrect** code for this rule:
  25
  26 ::: incorrect
  27
  28 ```js
  29 /*eslint no-useless-concat: "error"*/
  30 /*eslint-env es6*/
  31
  32 var a = `some` + `string`;
  33
  34 // these are the same as "10"
  35 var a = '1' + '0';
  36 var a = '1' + `0`;
  37 var a = `1` + '0';
  38 var a = `1` + `0`;
  39 ```
  40
  41 :::
  42
  43 Examples of **correct** code for this rule:
  44
  45 ::: correct
  46
  47 ```js
  48 /*eslint no-useless-concat: "error"*/
  49
  50 // when a non string is included
  51 var c = a + b;
  52 var c = '1' + a;
  53 var a = 1 + '1';
  54 var c = 1 - 2;
  55 // when the string concatenation is multiline
  56 var c = "foo" +
  57     "bar";
  58 ```
  59
  60 :::
  61
  62 ## When Not To Use It
  63
  64 If you don't want to be notified about unnecessary string concatenation, you can safely disable this rule.