为什么JavaScript for循环中的迭代器不需要分号？

Question

I'm curious why when making a for loop in JavaScript a ; is not necessary after the i++ statement? It tripped me up. It actually doesn't even work if you include a semicolon after the iterator statement.

eg

    for(var x=1; x<100; x++){

        document.write(x);
    }

Answer 1

While tokenizing the code for compiling, the JS-parser usually knows where statements start and end due to Automatic Semicolon Insertion .

In the case of a for-loop however, semicolons are interpreted as a separator of statements. For-Loops accept up to three "parameters". If you put in a semicolon, a fourth one will be expected and it will throw an error.

The final-expression -statement of a for-loop is terminated by ) .

This control-structure behaves similar to PHP , C , Java and C++

_{thanks to @Antoniu Livadariu for mentioning this in the comments}

Further Information

• MDN - Lexical Grammar
• MDN - The "for" Statement
• thejameskyle's super tiny compiler