Javascript – ny way to check if strict mode is enforced

The difference is that functionOne is a function expression and so only defined when that line is reached, whereas functionTwo is a function declaration and is defined as soon as its surrounding function or script is executed (due to hoisting).

For example, a function expression:

// TypeError: functionOne is not a function
functionOne();

var functionOne = function() {
  console.log("Hello!");
};

And, a function declaration:

// Outputs: "Hello!"
functionTwo();

function functionTwo() {
  console.log("Hello!");
}

Historically, function declarations defined within blocks were handled inconsistently between browsers. Strict mode (introduced in ES5) resolved this by scoping function declarations to their enclosing block.

'use strict';    
{ // note this block!
  function functionThree() {
    console.log("Hello!");
  }
}
functionThree(); // ReferenceError

2017 — 2021 update

Since 2009 when this question was asked, JavaScript has evolved significantly. All other answers are now obsolete or overly complicated. Here is the current best practice:

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function demo() {
  console.log('Taking a break...');
  await sleep(2000);
  console.log('Two seconds later, showing sleep in a loop...');

  // Sleep in loop
  for (let i = 0; i < 5; i++) {
    if (i === 3)
      await sleep(2000);
    console.log(i);
  }
}

demo();

This is it. await sleep(<duration>).

Or as a one-liner:

await new Promise(r => setTimeout(r, 2000));

Note that,

1. await can only be executed in functions prefixed with the async keyword, or at the top level of your script in an increasing number of environments.
2. await only pauses the current async function. This means it's not blocking the execution of the rest of the script, which is what you want in the vast majority of the cases. If you do want a blocking construct, see this answer using Atomics.wait, but note that most browsers will not allow it on the browser's main thread.

Two new JavaScript features (as of 2017) helped write this "sleep" function:

• Promises, a native feature of ES2015 (aka ES6). We also use arrow functions in the definition of the sleep function.
• The async/await feature lets the code explicitly wait for a promise to settle (resolve or reject).

Compatibility

• promises are supported in Node v0.12+ and widely supported in browsers, except IE
• async/await landed in V8 and has been enabled by default since Chrome 55 (released in Dec 2016)
  • it landed in Node 7 in October 2016
  • and also landed in Firefox Nightly in November 2016

If for some weird reason you're using Node older than 7 (which has reached end of life), or are targeting old browsers, async/await can still be used via Babel (a tool that will transpile JavaScript + new features into plain old JavaScript), with the transform-async-to-generator plugin.