Useful JavaScript Tips & Tricks

• Console Table - Display tabular data as a table.
• Console Log With Variable Name.
• Short code to create array of n length from 0 to n-1.
• Conditionally Spread Object.
• Conditionally adding properties inside Object.
• Type Checking JavaScript Files
• Easily trace execution time of Promises
• Check if an array with a length is not just empty slots

You might have come across a situation like this where you have to wade through a sea of array of objects just to check a particular object key value in the array for debugging purpose maybe. Something like this:

... and then you tried to guess where that object could be and kept opening and closing each object one by one, something like this:

In such cases, you can easily review the data in nice table format using console.table like:

// You have an array of object
var data = [{...},{...},{...},{...},{...},{...}];

// Just pass it to console.table
console.table( data );

and after that you will see something like this in your console:

which now you can easily scroll and check for object key value that you have been looking for.

MDN Web Docs: Console.table()

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There might have been a situation where you console.log some variables in app like:

console.log(id, name, completed, submitted);

and then check in console for results on page refresh and find something like:

23 "palash" true false

Now at first look you might have some confusion like "Is completed true or false here?" You just check the app/IDE again and confirm based on position of variables inside console.log that completed is actually true here and submitted variable is false. But you can just wrap the curly brackets {} around your console.log arguments to see the variable names along with the value in a easy comprehensible format, taking the advantage of ES6 shorthand syntax like:

console.log({ id, name, completed, submitted });

and now in your browser console you will see something like:

Now you can easily say that completed is indeed true here.

Reference: Wes Bos's 2016 Tweet

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Sometimes we might need to create an array of n length like [0, 1, 2, ..., n-1]. We can do this easily using ES6 Spread syntax like:

[...Array(n).keys()];

This is a general version. If you need an array of length 10, then you can set it like:

[...Array(10).keys()];
// Result => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

1. Array(10) create an empty array of length 10 like [,,,...] where value of any index from 0-9 is undefined.

2. Array(10).keys() this returns a new Array Iterator object that contains the keys for each index in the array.

   var array1 = Array(3);
   var iterator = array1.keys();
   
   for (let key of iterator) {
     console.log(key); // expected output: 0 1 2
   }

3. [...Array(10).keys()] this creates a new, shallow-copied Array instance from Array(10).keys() iterable object.

If you want the array to start from 1 and end with n here like [1, 2, 3, ..., n] instead of [0, 1, 2, ..., n-1], then you can do like this:

[...Array(10)].map((v, i) =>   i);
// Result => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

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We can conditionally spread an object - because {...false} (and undefined/null/etc) is just {}.

let person;
const defaultAddress = {
    street: '4253  Rebecca Street',
    city: 'Chicago',
    state: 'Illinois',
    country: 'US'
}

// In case, person object has not been set yet then person is `undefined`
const shipping = {
 	...defaultAddress,
	...person.address
}
// and the above code gives an error:
// Uncaught TypeError: Cannot read property 'address' of undefined

// We can conditionally spread the person object like:
const shipping = {
 	...defaultAddress,
	...(person && person.address)   // this will result in {...false} == {}
}

// and we will not get any type error incase person object has not been set yet

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We can return two different objects with or without certain property, based on a condition using the spread operator.

// A boolean `emailIncluded` determines whether the property `email` is added to the return object
const getUser = (emailIncluded) => {
  return {
    name: 'Wes',
    surname: 'Bos',
    ...(emailIncluded && { email : '[email protected]' })
  }
}

// Get user details with email
const user = getUser(true);
console.log(user);
// Returns:
// {name: "Wes", surname: "Bos", email: "[email protected]"}

// Get user details without email
const userWithoutEmail = getUser(false);
console.log(userWithoutEmail);
// Returns:
// {name: "Wes", surname: "Bos"}

The spread operator for object does nothing if its operand is an empty object

{a: 1, ...{}}
// Returns:
// {a: 1}

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Want TypeScript's type checking for regular JavaScript?

Opt in via comment: // @ts-check
Via CLI: tsc file.js --allowJs --checkJs --noEmit
Via config: "checkJs": true

If types can't be inferred, use JSDoc annotations. Learn more: Link

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Easily trace execution time of Promises with a combo of await & finally

const delay = require("delay");

trace("How long?", async () => {
  await delay(500);
});

// Adapted from https://github.com/apollographql/apollo-server/blob/d5015f4ea00cadb2a74b09956344e6f65c084629/packages/apollo-datasource-rest/src/RESTDataSource.ts#L281
async function trace(label, fn) {
  if (process && process.env && process.env.NODE_ENV === "development") {
    const startTime = Date.now();
    try {
      return await fn();
    } finally {
      const duration = Date.now() - startTime;
      console.log(`${label} (${duration}ms)`);
    }
  } else {
    return fn();
  }
}

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We can determine if an array with a length is not just empty slots, using array filter() method. The filter() method creates a new array with all elements that pass the test implemented by the provided function.

// Make an array with 4 empty slots
Array(4)
=> (4) [empty × 4]

// Check this array length
Array(4).length
=> 4

// But does all of the array slots have a valid value
// 0 here means that all of the slots of this array are empty
Array(4).filter(Boolean).length
=> 0

// To verify we can create an array with two empty slots and 2 valid slots
Array(undefined, undefined, 3, 5)
=> (4) [undefined, undefined, 3, 5]

// Check this array length
Array(undefined, undefined, 3, 5).length
=> 4

// But does all of the array slots have a valid value
// 2 here means that only two slots have a valid value in this array
Array(undefined, undefined, 3, 5).filter(Boolean).length
=> 2

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Coming soon...