​Language element

Restriction

Error

Example

Variable

Using a variable without declaring it.

SCRIPT5042: Variable undefined in strict mode

JavaScript

testvar = 4;

Read-only property

Writing to a read-only property.

SCRIPT5045: Assignment to read-only properties is not allowed in strict mode

JavaScript

var testObj = Object.defineProperties({}, {
    prop1: {
        value: 10,
        writable: false // by default
    },
    prop2: {
        get: function () {
        }
    }
});
testObj.prop1 = 20; 
testObj.prop2 = 30;

Non-extensible property

Adding a property to an object whoseextensibleattribute is set to false.

SCRIPT5046: Cannot create property for a non-extensible object

JavaScript

var testObj = new Object();

Object.preventExtensions(testObj);

testObj.name = "Bob";

delete

Deleting a variable, a function, or an argument.

Deleting a property whoseconfigurableattribute is set to false.

SCRIPT1045: Calling delete on <expression>is not allowed in strict mode

JavaScript

var testvar = 15;function testFunc() {};delete testvar;delete testFunc;

Object.defineProperty(testObj, "testvar", {
    value: 10,
    configurable: false
    });delete testObj.testvar;

Duplicating a property

Defining a property more than once in an object literal.

SCRIPT1046: Multiple definitions of a property not allowed in strict mode

JavaScript

var testObj = {
    prop1: 10,
    prop2: 15,
    prop1: 20
};

Duplicating a parameter name

Using a parameter name more than once in a function.

SCRIPT1038: Duplicate formal parameter names not allowed in strict mode

JavaScript

function testFunc(param1, param1) {    return 1;
};

Future reserved keywords

Using a future reserved keyword as a variable or function name.

SCRIPT1050: The use of a future reserved word for an identifier is invalid. The identifier name is reserved in strict mode.

• implements

• interface

• package

• private

• protected

• public

• static

• yield

Octals

Assigning an octal value to a numeric literal, or attempting to use an escape on an octal value.

SCRIPT1039: Octal numeric literals and escape characters not allowed in strict mode

JavaScript

var testoctal = 010;var testescape = \010;

this

The value ofthis is not converted to the global object when it is null orundefined.

JavaScript

function testFunc() {
    return this;
}
var testvar = testFunc();

In non-strict mode, the value of testvar is the global object, but in strict mode the value is undefined.

evalas an identifier

The string "eval" cannot be used as an identifier (variable or function name, parameter name, and so on).

JavaScript

var eval = 10;

Function declared inside a statement or a block

You cannot declare a function inside a statement or a block.

SCRIPT1047: In strict mode, function declarations cannot be nested inside a statement or block. They may only appear at the top level or directly inside a function body.

JScript

var arr = [1, 2, 3, 4, 5];
var index = null;
for (index in arr) {
    function myFunc() {};
}

Variable declared inside an evalfunction

If a variable is declared inside anevalfunction, it cannot be used outside that function.

SCRIPT1041: Invalid usage of 'eval' in strict mode

JavaScript

eval("var testvar = 10");
testvar = 15;

Indirect evaluation is possible, but you still cannot use a variable declared outside the eval function.

JavaScript

var indirectEval = eval;
indirectEval("var testvar = 10;");
document.write(testVar);

This code causes an error SCRIPT5009: 'testVar' is undefined.

Argumentsas an identifier

The string "arguments" cannot be used as an identifier (variable or function name, parameter name, and so on).

SCRIPT1042: Invalid usage of 'arguments' in strict mode

JavaScript

var arguments = 10;

argumentsinside a function

You cannot change the values of members of the localargumentsobject.

JavaScript

function testArgs(oneArg) {
    arguments[0] = 20;
}

In non-strict mode, you can change the value of the oneArgparameter by changing the value of arguments[0], so that the value of both oneArg and arguments[0] is 20. In strict mode, changing the value of arguments[0] does not affect the value of oneArg, because the arguments object is merely a local copy.

arguments.callee

Not allowed.

JavaScript

function (testInt) {
    if (testInt-- == 0)
        return;
    arguments.callee(testInt--);
}

with

Not allowed.

SCRIPT1037: 'with' statements are not allowed in strict mode

JavaScript

with (Math){
    x = cos(3);
    y = tan(7);
}