JS：为什么允许使用和不使用`var`关键字来声明变量？

Question

It is possible to declare a javascript variable both with and without the var keyword.

var a = 100; // this works!
    b = 200; // and this does too!

It is also possible to declare a variable without initialisation.

var c;       // this is just as acceptable! 

But then why is the same NOT true for a variable without var, to be declared without initialisation.

var c; 
    d;       // causes a reference error to occur!

Why?

Answer 1

First, what you're seeing is legacy behavior. Assignment to an undeclared symbol traditionally meant, implicitly, that a global symbol should be created (declared) and set to the given value. Thus

x = 1;

when x has not been declared was taken to be an implicit instantiation of a global symbol.

The mention of an undeclared symbol, as in:

x;

is an error because the symbol is undeclared.

In modern JavaScript, and when "strict" mode is in force because of a

"use strict";

statement (or because of other influences, as may be the case with Node.js code), the implicit creation of global symbols is also erroneous.

Generally, implicit global symbol instantiation is considered a bad idea. Global symbols in browser JavaScript are quite problematic because the global namespace is so horribly polluted. Thankfully, it's easy to wrap code in a function scope to create a "safe space" for symbols without fear of the browser imposing weird global names.

Answer 2

You can do that in non-strict mode

   var a = 100; // this works!
        b = 200; // and this does too!

for non-strict mode, someVar = someValue, if someVar is not existed, javascript will declare it and assign a someValue to someVar.

For that case:

var c; 
    d;       // causes a reference error to occur!

line 1: var c; --> declare c, it's valid syntax. line 2: d; --> you access d, but d is underfined --> causes error !!! assume d is defined anywhere before that line, so it should NOT cause error!!