Chrome Dev Tools Tricks

Exercises to practise using the Chrome developer tools. Part of Wes Bos' JavaScript 30 course.

Quick debugging

To see what JavaScript is modifying an element, inspect the element, find it in the DOM, right click, click on 'break on...' and 'attribute modifications'.

Now when you click on the element and trigger the JavaScript, a debugger command is run. It pauses exactly when the element is modified and shows the line of code that is causing the element's attribute to be modified.

Console methods and more

These also work in Firefox's console.

Regular

console.log outputs to the console.

Interpolated

You can interpolate arguments into console.log

console.log("Hello, I am a %s string", "test");

To some extent this has been replaced by ES6 backticks.

console.log(Hello, I am a ${var} string);

Styled

You can style text in the console with %c:

console.log('%c I am some great text', 'font-size: 50px; background: red;');

Table

console.table() displays data (e.g. an array of objects) as a table.

Warning

console.warn() outputs a warning message.

console.warn("OH NOOOOO!!");

Error

console.error() outputs an error message to the console.

console.error("Feck!");

Info

console.info() outputs an informational message to the console (marked with an "i" icon).

console.info("Crocodiles eat 3-4 people per year");

Testing

console.assert() checks if things are true. It writes an error message to the console if the assertion is false.

console.assert(1 === 2, "That is wrong");
const p = document.querySelector("p");
console.assert(p.classList.contains("ouch"), "That is wrong");

Clear

console.clear() will clear your console.

Viewing DOM elements

console.dir() gives you an interactive list of the properties and methods of the element you pass to it.

Grouping things together

console.group() creates a new inline group, indenting all following output by another level.

console.groupCollapsed() also creates an inline group but starts with the group collapsed.

console.groupEnd() moves back out a level.

dogs.forEach(dog => {
  console.groupCollapsed(`${dog.name}`);
  console.log(`This is ${dog.name}`);
  console.log(`${dog.name} is ${dog.age} years old`);
  console.log(`${dog.name} is ${dog.age * 7} dog years old`);
  console.groupEnd(`${dog.name}`);
});

Counting

console.count() logs the number of times you call that line with a specific label.

Timing

console.time() starts a timer.

console.timeEnd() stops the specified timer.

console.time("Fetching data");
fetch("https://api.github.com/users/wesbos")
  .then(data => data.json())
  .then(data => {
    console.timeEnd("Fetching data");
  });