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20. strict mode.md

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20-01

function foo() {
  x = 10;
}
foo();

console.log(x); // ?

20-02

'use strict';

function foo() {
  x = 10; // ReferenceError: x is not defined
}
foo();

20-03

function foo() {
  'use strict';

  x = 10; // ReferenceError: x is not defined
}
foo();

20-04

function foo() {
  x = 10; // 에러를 발생시키지 않는다.
  'use strict';
}
foo();

20-05

<!DOCTYPE html>
<html>
<body>
  <script>
    'use strict';
  </script>
  <script>
    x = 1; // 에러가 발생하지 않는다.
    console.log(x); // 1
  </script>
  <script>
    'use strict';

    y = 1; // ReferenceError: y is not defined
    console.log(y);
  </script>
</body>
</html>

20-06

// 즉시 실행 함수의 선두에 strict mode 적용
(function () {
  'use strict';

  // Do something...
}());

20-07

(function () {
  // non-strict mode
  var lеt = 10; // 에러가 발생하지 않는다.

  function foo() {
    'use strict';

    let = 20; // SyntaxError: Unexpected strict mode reserved word
  }
  foo();
}());

20-08

(function () {
  'use strict';

  x = 1;
  console.log(x); // ReferenceError: x is not defined
}());

20-09

(function () {
  'use strict';

  var x = 1;
  delete x;
  // SyntaxError: Delete of an unqualified identifier in strict mode.

  function foo(a) {
    delete a;
    // SyntaxError: Delete of an unqualified identifier in strict mode.
  }
  delete foo;
  // SyntaxError: Delete of an unqualified identifier in strict mode.
}());

20-10

(function () {
  'use strict';

  //SyntaxError: Duplicate parameter name not allowed in this context
  function foo(x, x) {
    return x + x;
  }
  console.log(foo(1, 2));
}());

20-11

(function () {
  'use strict';

  // SyntaxError: Strict mode code may not include a with statement
  with({ x: 1 }) {
    console.log(x);
  }
}());

20-12

(function () {
  'use strict';

  function foo() {
    console.log(this); // undefined
  }
  foo();

  function Foo() {
    console.log(this); // Foo
  }
  new Foo();
}());

20-13

(function (a) {
  'use strict';
  // 매개변수에 전달된 인수를 재할당하여 변경
  a = 2;

  // 변경된 인수가 arguments 객체에 반영되지 않는다.
  console.log(arguments); // { 0: 1, length: 1 }
}(1));