Expressions versus statements in JavaScript
Statements and expressions
An expression produces a value and can be written wherever a value is expected.
Expressions that look like statements
Expressions that look like statements
JavaScript has stand-alone blocks? It might surprise you that JavaScript has blocks that can exist on their own (as opposed to being part of a loop or an if statement). The following code illustrates one use case for such blocks: You can give them a label and break from them.
function test(printTwo) {
printing: {
console.log("One");
if (!printTwo) break printing;
console.log("Two");
}
console.log("Three");
}
Function expression versus function declaration
function expressions
function() {}
function fn() {}
Using object literals and function expressions as statements
eval
parses its argument in statement context. If you wanteval
to return an object, you have to put parentheses around an object literal.> eval("{ foo: 123 }")
123
> eval("({ foo: 123 })")
{ foo: 123 }
Immediately invoked function expressions (IIFEs)
> (function () { return "abc" }())
'abc'
If you omit the parentheses, you get a syntax error (function declarations can’t be anonymous):
> function () { return "abc" }()
SyntaxError: function statement requires a name
If you add a name, you also get a syntax error (function declarations can’t be immediately invoked):
> function foo() { return "abc" }()
SyntaxError: syntax error
Another way of guaranteeing that an expression is parsed in expression context is a unary operator such as + or !. But, in contrast to parentheses, these operators change the result of the expression. Which is OK, if you don’t need it:
> +function () { console.log("hello") }()
hello
NaN
NaN
is the result of applying + to undefined, the result of calling the function. Brandon Benvie mentions another unary operator that you can use: void [2]:> void function () { console.log("hello") }()
hello
undefined
Concatenating IIFEs
When you concatenate IIFEs, you must be careful not to forget semicolons:
(function () {}())
(function () {}())
// TypeError: undefined is not a function
This code produces an error, because JavaScript thinks that the second line is an attempt to call the result of the first line as a function. The fix is to add a semicolon:
(function () {}());
(function () {}())
// OK
With operators that are only unary (plus is both unary and binary), you can omit the semicolon, because automatic semicolon insertion kicks in.
void function () {}()
void function () {}()
// OK
JavaScript inserts a semicolon after the first line, because void is not a valid way of continuing this statement [3].
原文:Expressions versus statements in JavaScript
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