What is JavaScript’s strict mode?

"use strict";
let mistypeVariable;

// Assuming no global variable mistypeVarible exists
// this line throws a ReferenceError due to the
// misspelling of "mistypeVariable" (lack of an "a")
mistypeVarible = 17;

"use strict";

// Assignment to a non-writable global
var undefined = 5; // TypeError
var Infinity = 5; // TypeError

// Assignment to a non-writable property
const obj1 = {};
Object.defineProperty(obj1, "x", { value: 42, writable: false });
obj1.x = 9; // TypeError

// Assignment to a getter-only property
const obj2 = {
  get x() {
    return 17;
  },
};
obj2.x = 5; // TypeError

// Assignment to a new property on a non-extensible object
const fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = "ohai"; // TypeError

"use strict";

function fun() {
  return this;
}
console.assert(fun() === undefined);

"use strict";
delete Object.prototype; // TypeError
delete [].length; // TypeError

var x = 17;
var evalX = eval("'use strict'; var x = 42; x;");
console.assert(x === 17);
console.assert(evalX === 42);

function sum(a, a, c) {
  // syntax error
  "use strict";
  return a + a + c; // wrong if this code ran
}

"use strict";

false.true = ""; // TypeError
(14).sailing = "home"; // TypeError
"with".you = "far away"; // TypeError

// Whole-script strict mode syntax
"use strict";
const v = "Hi! I'm a strict mode script!";

function myStrictFunction() {
  // Function-level strict mode syntax
  "use strict";
  function nested() {
    return "And so am I!";
  }
  return `Hi! I'm a strict mode function! ${nested()}`;
}
function myNotStrictFunction() {
  return "I'm not strict.";
}