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Editorial Team
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Asked: 2026-06-18T02:18:53+00:00

This works. var a = ‘ontouchstart’ in window; for (;;) { console.log(a); break; }

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This works.

var a = 'ontouchstart' in window;
for (;;) {
  console.log(a);
  break;
}

This causes syntax error. Why?

for (var a = 'ontouchstart' in window;;) {
  console.log(a);
  break;
}

This works.

for (var a = ('ontouchstart' in window);;) {
  console.log(a);
  break;
}
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1 Answer

  1. Editorial Team

    This causes syntax error. Why?

    To avoid confusion with for-in-loops. The syntax specification for for-loops is explicit:

    IterationStatement : for ( ExpressionNoInopt; Expressionopt; Expressionopt) Statement

    IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt; Expressionopt) Statement

    This NoIn suffix spreads through the whole syntactic grammar, and ends in the 11.8 Relational Operators (Syntax) section:

    RelationalExpression :

ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression

RelationalExpressionNoIn :

ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression

    NOTE: The NoIn variants are needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

    However, I don’t understand myself why the NoIn variants are used in the normal for-loop – they are reasonable in for-in-productions. I’d guess it’s to avoid confusion of the programmer and to simplify parsers.

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