no-unexpected-multiline.md

no-unexpected-multiline

• When all of the following conditions are met, throw error.

  1. The node is + expression or - expression.
  2. There is line-break before the operator.
  3. The line starts with an operator is not indented.

• On error, the --fix option inserts ; before the operator.

Rule Details

• Here's a summary of why this rule is required.

Unexpected Multiline Expression

• Code without semicolons may produce unexpected multi-line expressions.

• Examples

// 1.
MyClass.prototype.myMethod = function() {
  return 42;
}  // No semicolon here.

(function() {
  // Some initialization code
  // wrapped in a function to
  // create a scope for locals.
})();


var x = {
  'i': 1,
  'j': 2
}  // No semicolon here.

// 2. Trying to do one thing on Internet Explorer and another on Firefox.
[ffVersion, ieVersion][isIE]();


var THINGS_TO_EAT = [apples, oysters, sprayOnCheese]  // No semicolon here.

// 3. conditional execution a la bash
-1 == resultOfOperation() || die();

• In the above example, 1 and 2 do not cause problems.

• 3. causes a problem where an expression beginning with -1 is connected to the previous line and recognized as a "subtraction expression".

How to Resolve

• Regarding this issue, we have considered as following.

  1. A problem arose where a unary operator expression like -1 written at the beginning of a line was recognized as "a subtraction expression following from the previous line".
  2. If a programmer "writes a line that begins with a unary operator without indentation," it can be determined that this is not the intention of "code written as a continuation of the previous line.
  3. Tell the programmer that there is an "unexpected multi-line expression" on the relevant line by raising an error in ESLint that the indentation is incorrect.

• When resolving with --fix, the ; is inserted immediately before the second line operator to eliminate "the possibility of unexpected multi-line expressions".

Version

• Since 0.1.0