这很有效.
var a = 'ontouchstart' in window;
for (;;) {
console.log(a);
break;
}
这会导致语法错误.为什么？
for (var a = 'ontouchstart' in window;;) {
console.log(a);
break;
}
这很有效.
for (var a = ('ontouchstart' in window);;) {
console.log(a);
break;
}
解决方法
This causes Syntax error. Why?
避免与for-in-loops混淆. syntax specification for for-loops是明确的：
IterationStatement : for ( ExpressionNoInopt; Expressionopt; Expressionopt) Statement
IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt; Expressionopt) Statement
这个NoIn后缀通过整个语法语法传播,并在11.8 Relational Operators (Syntax) section结束：
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
NoInvariants are needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.
但是,我不明白为什么在正常的for循环中使用NoIn变体 – 它们在制作中是合理的.我猜这是为了避免程序员混淆并简化解析器.
