ringa_lee2017-05-19 10:36:20
Q1
Press Enter, the console will use the js parser of the page to execute the entered code, and display the return value obtained by executing the last statement in the console.
var a=1 // undefined
a=111 //111
111 // 111
The reason is that the return value of the var statement is undefined, a=111这是一个赋值运算，赋值运算的返回值是a. It is a bit difficult to explain the error reported by the anonymous function.
There are two concepts about functions in js: function declaration and function expression.
var a = function(){} // 函数表达式,（匿名函数表达式）
function b(){} // 函数声明
var c = function c(){} // 函数表达式（命名函数表达式）
// 旧IE不支持named function expression
Since function declarations are very similar to named function expressions, how does the js engine distinguish function whether a keyword is followed by a function declaration or a function expression? The answer lies in the current context. If the context is an evaluation environment, then function is considered to introduce a function expression, otherwise it is considered to be a function declaration.
The error reported in Q1 is because the current context is not an evaluation context, and function is parsed as a function declaration. The error Uncaught SyntaxError: Unexpected token ( means an unexpected left bracket. It is easy to understand. The function declaration requires a function name, which is not given here. A left bracket appears in the function name, so the syntax error reported is in the position of the left bracket.
Q2
, Comma is an operator in js. The comma operator will execute the expressions around the comma in sequence, and the return value is the value of the expression on the right side of the comma.
1,function(){} // => function(){}
The reason why no error is reported is because the context 1,... constitutes an evaluation context, so the subsequent function is correctly parsed as a function expression.
The error is reported when writing in reverse, because the comma operator has not been parsed yet, and it is not currently an evaluation context, so function is recognized as a function declaration. As for why we don’t backtrack after parsing the comma, this may be due to implementation costs. There are many operators for left and right operands, and for example:
function a(){}
+“”
Do you think this kind of code should be parsed into a named function expression plus an empty string, or should it be parsed into a function declaration and a statement?
So, judging from the phenomenon, when judging the function keyword, the js engine only looks at the current context without backtracking. (There may be other potential problems that make backtracking impossible)
Q3
()括号在 js 中也是一种运算符，叫组运算符, the group operator has the highest priority, and the return value is the result of the expression evaluation within the brackets.
(function(){})构成了一个合法的表达式，而(var a=1)报错是因为组运算符内只允许表达式，var a=1 is a statement.
Q4
(0,function(){
console.log(1)
})() // log 1, 返回 undefined
(function(){
console.log(1)
}, 2)() // 报错，2不是函数
I believe you already understand.
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