javascript - 为什么这个javascript在严格模式下没有错误

Question

这很奇怪。我放置“严格使用”的地方会有不同的结果。
我的node版本是v9.9.0
我不明白，谁能帮助我

"use strict";

function tryFunction() {
  var tryValue = 123;
  return tryValue;
}
if (true) {
  testvar = 123; // ReferenceError: testvar is not defined
}

function tryFunction() {
  var tryValue = 123;
  return tryValue;
}
"use strict";
if (true) {
  testvar = 123;
}

// no errors???

function tryFunction() {
  var tryValue = 123;
  return tryValue;
}
if (true) {
  "use strict";
  testvar = 123;
}

// no errors???

Answer 1

参见例如MDN documentation for strict mode :

Strict mode applies to entire scripts or to individual functions.

[...]

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.

[...]

Likewise, to invoke strict mode for a function, put the exact statement "use strict"; (or 'use strict';) in the function's body before any other statements.

(强调我的。)

如果 "use strict"; 出现在文件或 block 的中间，它没有任何效果，会像任何其他字符串文字一样被忽略。