规范的相关部分:

http://www.ecma-international.org/ecma-262/5.1/#sec-10.1.1

其中说:

在以下情况下，

Code is interpreted as strict mode code in the following situations:

• 如果全局代码以包含使用严格指令(请参见14.1)的指令序言开头，则它是严格的全局代码.

  • Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1).

    评估代码以包含使用严格指令的指令序言开头或调用eval，则该评估代码为严格的评估代码是对eval函数的直接调用(请参见15.1.2.1.1)，即包含在严格模式代码中.

    Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is contained in strict mode code.

    属于FunctionDeclaration，FunctionExpression或访问器PropertyAssignment的函数代码是严格函数代码，如果其FunctionDeclaration，FunctionExpression或PropertyAssignment包含在严格模式代码中，或者如果函数包含代码以包含使用严格性的指令序言开头指令.

    Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.

    作为最后一个参数提供给内置Function构造函数的函数代码是严格的函数代码，如果最后一个参数是作为FunctionBody处理时以指令开头的字符串包含使用严格指令的序言.

    Function code that is supplied as the last argument to the built-in Function constructor is strict function code if the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

    因此，对于在严格范围"内明确定义的函数，它们将继承严格模式:

    So for functions defined explicitly within a 'strict scope', they will inherit strict mode:

    function doSomethingStrict(){
        "use strict";
    
        // in strict mode
    
        function innerStrict() {
            // also in strict mode
        }
    }
    

    但是使用 Function 构造函数创建的函数不会从其上下文继承严格模式，因此必须具有明确的"use strict"; 声明是否要在严格模式下使用.例如，请注意 eval 在严格模式下(但不是在严格模式下)是保留关键字:

    But functions created using the Function constructor don't inherit strict mode from their context, so must have an explicit "use strict"; statement if you want them in strict mode. For example, noting that eval is a reserved keyword in strict mode (but not outside of strict mode):

    "use strict";
    
    var doSomething = new Function("var eval = 'hello'; console.log(eval);");
    
    doSomething(); // this is ok since doSomething doesn't inherit strict mode