Return to Answer

• Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)

• In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)

• In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.

• In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.

• In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.

• In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).

• In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)

• In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)

• In strict mode octal literals are not allowed (these are literals that start with 0x. (In normal mode some implementations do allow octal literals.)

• In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.

• In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.

• Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)

• In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)

• In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.

• In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.

• In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.

• In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).

• In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)

• In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)

• In strict mode octal literals are not allowed (these are literals that start with 0. (In normal mode some implementations do allow octal literals.)

• In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.

• In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.

"use strict" a directive introduced in ECMAScript 5.

"use strict" a directive introduced in ECMAScript 5.

I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive as I could.

I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive and complete as I could.