语法错误，还是没有语法错误？

Question

var zero = 0;
zero.toString();   // '0' --> fine
0.toString();      // syntax error!
0..toString();     // '0' --> fine

My conclusion: calling x.toString() doesn't only depend on the value of x , but also on the way x is presented .

Are there other such examples in JavaScript where I might get an unexpected syntax error due to presentation?

Answer 1

Well, there are other cases where the context of where symbols appear affects how they behave, for example, statement Block's vs Object Literals:

{}          // empty block
var o = {}; // empty object
({});       // empty object
0,{}        // empty object

{ foo: 'bar'} // block, `foo` label, 'bar' ExpressionStatement
var o = { foo: 'bar'}; // object literal, declaring a `foo` property
                       // whose value is 'bar'

They look exactly the same, but blocks are evaluates in "statement context", object literals are evaluated in expression context .

Also, Function Declarations vs. Function Statements, eg:

function foo() {}.length;   // SyntaxError, `foo` is a function declaration
0,function foo() {}.length; // 0, `foo` is a function expression
(function foo {}).length;   // 0

The example you post, relates to the way the grammar of Numeric literals are defined, the decimal part after the dot is actually optional, for example:

var n = 0.;

Is a valid Numeric literal, that's why, accessing 0.toString gives you a SyntaxError , the interpreter would expect the decimal part, instead the s character.

See also:

• What does 1..something mean in JavaScript?

Answer 2

The value of a variable doesn't matter, but the way it is "presented" might make it valid or invalid syntax. See this highly upvoted answer . It's just an oddity of the EcmaScript syntax definition.