Strict Mode Back

Strict Mode(嚴格模式) is a way in ES5 to opt in to a restricted variant of JavaScript.Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode.

Strict Mode has made some changes to normal semantics:

  • Strict mode eliminates(消除) some JavaScript silent errors by changing them to throw errors.
  • Strict mode fixes mistakes that make it difficult for JavaScript engines to perform optimization: strict mode code can sometimes be made to run faster than identical code that's not strict mode.
  • Strict mode prohibits(禁止) some syntax likely to be defined in future versions of ECMAScript.

Use Strict Mode in a script

Just add a line of use strict; like:

"use strict";
/** ... */

Changes

Converting mistakes into errors

First, strict mode makes it impossible to accidentally create global variables. In normal JavaScript mistyping a variable in an assignment creates a new property on the global object and continues to "work" (although future failure is possible: likely, in modern JavaScript). Assignments which would accidentally create global variables instead throw in strict mode:

"use strict";
mistypedVaraible = 17;  /** 
                         * Assuming a global variable mistypedVariable exists
                         * this line throws a ReferenceError due to the 
                         * misspelling of variable
                         */

Second, strict mode makes assignments which would otherwise silently fail throw an exception.

"use strict";

/** Assignment to a non-writable property */
var obj1 = {};
Object.defineProperty(obj1, "x", {
    value: 42,
    writable: false
});
/** throws a TypeError */
obj1.x = 9;

/** Assignment to a getter-only property */
var obj2 = {
    get x() { return 17; }
};
/** throws a TypeError */
obj2.x = 5;

/** Assignment to a new property on a non-extensible object */
var fixed = {};
Object.preventExtensions(fixed);
/** throws a TypeError */
fixed.newProp = 'ohai';

Third, strict mode makes attempts to delete undeletable properties throw (where before the attempt would simply have no effect):

"use strict";
/** throws a TypeError */
delete Object.prototype;

Fourth, strict mode prior to Gecko 34 requires that all properties named in an object literal be unique. (No longer the case in ES6: bug 1041128)

"use strict";
/** throws a SyntaxError */
var obj = {
    p: 1,
    p: 2
};

Fifth, strict mode requires that function parameter names be unique.

/** throws a SyntaxError */
function duplicate(a, b, a) {
    "use strict";
    return a + b + a;
}

Sixth, strict mode in ECMAScript 5 forbids octal syntax. Octal syntax isn't part of ECMAScript 5, but it's supported in all browsers by prefixing the octal number with a zero: 0644 === 420 and "\045" === "%". In ECMAScript 6 Octal number is supported by prefixing a number with "0o". i.e.

/** supported in ES6 */
var a = 0o10;

Seventh, strict mode in ECMAScript 6 forbids setting properties on primitive values:

"use strict";
/** throws a TypeError */
false.true = '';
/** throws a TypeError */
(14).sell = true;
/** throws a TypeError */
'abc'.what = 'what';

Simplifying variable uses

First, strict mode prohibits with. The problem with with is that any name inside the block might map either to a property of the object passed to it, or to a variable in surrounding (or even global) scope, at runtime: it's impossible to know which beforehand. Strict mode makes with a syntax error, so there's no chance for a name in a with to refer to an unknown location at runtime:

"use strict";
var x = 17;
/** throws a SyntaxError */
with (obj) {
    console.log(x); /** 
                     * If this weren't strict mode, would this be var x, or
                     * would it instead be obj.x?  It's impossible in general
                     * to say without running the code, so the name can't be
                     * optimized.
                     */
}

Second, eval of strict mode code does not introduce new variables into the surrounding scope. In normal code eval("var x;") introduces a variable x into the surrounding function or the global scope. In strict mode eval creates variables only for the code being evaluated, so eval can't affect whether a name refers to an outer variable or some local variable:

var x = 17;
var evalX = eval('"use strict"; var x = 42; x');
console.assert(x === 17);
console.assert(evalX === 42);

Strict mode passing:

function strict1(str) {
    "use strict";
    /** str will be treated as strict mode code */
    return eval(str);
}

function strict2(f, str) {
    "use strict";
    return f(str);  /**
                     * not eval(...): str is strict if and only
                     * if it invokes strict mode
                     */
}

function nonstrict(str){
    return eval(str);   /**
                         * str is strict if and only 
                         * if it invokes strict mode
                         */
}

strict1("'Strict mode code!'");
strict1("'use strict'; 'Strict mode code!'");
strict2(eval, "'Non-strict code.'");
strict2(eval, "'use strict'; 'Strict mode code!'");
nonstrict("'Non-strict code.'");
nonstrict("'use strict'; 'Strict mode code!'");

Third, strict mode forbids deleting plain names. delete name in strict mode is a syntax error:

"use strict";

var x;
/** throws a SyntaxError */
delete x;
/** throws a SyntaxError */
eval("var y; delete y;");

Making eval and arguments simpler

First, the names eval and arguments can't be bound or assigned in language syntax. All these attempts to do so are syntax errors.

"use strict";
/** throws a SyntaxError */
var eval;
/** throws a SyntaxError */
try { } catch (arguments) { }

Second, strict mode code doesn't alias properties of arguments objects created within it. In normal code within a function whose first argument is arg, setting arg also sets arguments[0], and vice versa (unless no arguments were provided or arguments[0] is deleted). arguments objects for strict mode functions store the original arguments when the function was invoked. arguments[i] does not track the value of the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i].

function f(a) {
    "use strict";
    a = 42;
    return [a, arguments[0]];
}

var pair = f(17);
console.assert(pair[0] === 42);
console.assert(pair[1] === 17);

Third, arguments.callee is no longer supported.

"use strict";
var f = function () {
    return arguments.callee;
};
/** throws a TypeError */
f();

"Securing" Javascript

Strict mode makes it easier to write "secure" JavaScript.

First, the value passed as this to a function in strict mode is not forced into being an object (a.k.a. "boxed"). For a normal function, this is always an object: either the provided object if called with an object-valued this; the value, boxed, if called with a Boolean, string, or number this; or the global object if called with an undefined or null this. (Use call, apply, or bind to specify a particular this.) Not only is automatic boxing a performance cost, but exposing the global object in browsers is a security hazard(危害), because the global object provides access to functionality that "secure" JavaScript environments must restrict.

"use strict";
function fun() { return this; }
console.assert(fun() === undefined);
console.assert(fun.call(2) === 2);
console.assert(fun.apply(null) === null);
console.assert(fun.call(undefined) === undefined);
console.assert(fun.bind(true)() === true);

That means, among other things, that in browsers it's no longer possible to reference the window object through this inside a strict mode function.

Second, in strict mode it's no longer possible to "walk" the JavaScript stack via commonly-implemented extensions to ECMAScript. In normal code with these extensions, when a function fun is in the middle of being called, fun.caller is the function that most recently called fun, and fun.arguments is the arguments for that invocation of fun.

function restricted() {
    "use strict";
    /** throws a TypeError */
    restricted.caller;
    /** throws a TypeError */
    restricted.arguments;
}

function privilegedInvoker() {
    return restricted();
}

privilegedInvoker();

Third, arguments for strict mode functions no longer provide access to the corresponding function call's variables. In some old ECMAScript implementations arguments.caller was an object whose properties aliased variables in that function. This is a security hazard because it breaks the ability to hide privileged values via function abstraction; it also precludes(阻斷) most optimization. For these reasons no recent browsers implement it. Yet because of its historical functionality, arguments.caller for a strict mode function is also a non-deletable property which throws when set or retrieved:

"use strict";
function fun(a, b) {
    "use strict";
    var v = 12;
    /** throws a TypeError */
    return arguments.caller;
}

/** doesn't expose v (or a or b) */
fun(1, 2);

Paving the way(鋪路) for future ECMAScript versions

Future ECMAScript versions will likely introduce new syntax, and strict mode in ECMAScript 5 applies some restrictions to ease the transition.

First, in strict mode a short list of identifiers become reserved keywords. These words are implements, interface, let, package, private, protected, public, static, and yield. In strict mode, then, you can't name or use variables or arguments with these names.

Second, strict mode prohibits function statements not at the top level of a script or function. In normal code in browsers, function statements are permitted "everywhere". This is not part of ES5 (or even ES3)! It's an extension with incompatible semantics in different browsers. Future ECMAScript editions will hopefully specify new semantics for function statements not at the top level of a script or function. Prohibiting such function statements in strict mode "clears the deck" for specification in a future ECMAScript release:

"use strict";
if (true) {
    /** throws a SyntaxError */
    function f() { }
    f();
}

for (var i = 0; i < 5; i++){
    /** throws a SyntaxError */
    function f2() { }
    f2();
}

/** kosher(合理的) */
function baz() {
    /** kosher(合理的) */
    function eit() { }
}

See Also

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