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10 JavaScript Array/Object Tricks

318097 profile image Mehul Lakhanpal Originally published at Medium ・2 min read

1. Initialize an array of size n and fill with default values

const size = 5;
const defaultValue = 0;
const arr = Array(size).fill(defaultValue);
console.log(arr); // [0, 0, 0, 0, 0]
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2. Insert something in middle of array

const arr = [1, 2, 3, 4];
const index = 2;
const insertText = "hello";

// Solution 1 - Split at the index and rejoin them
const result = [...arr.slice(0, index), insertText, ...arr.slice(index)];
console.log(result); // [1, 2, 'hello', 3, 4]

// Solution 2 - .splice() - It can be used to add/remove elements from array
const arrCopy = [...arr]; // splice modified the array on which the operation is performed.
arrCopy.splice(index, 0, insertText); // arguments are - index, no of elements to remove, new element to add
console.log(arrCopy); // [ 1, 2, 'hello', 3, 4 ]
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3. Pick random element from an array

const themes = ['neo', 'black & white', 'color'];

const randomNumber =  Math.round(Math.random() * 100); // random number between 0 - 100
const randomElement = randomNumber % themes.length; // so that the number is within the arrays range 
console.log(themes[randomElement]);
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4. Check if a value is an array

const arr = [1, 2, 3];
console.log(typeof arr); // object
console.log(Array.isArray(arr)); // true
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5. Remove duplicates from array

const array = [1, 1, 2, 3, 5, 5, 1];
const uniqueArray = [...new Set(array)];

console.log(uniqueArray); // [1, 2, 3, 5]
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6. Check if an object is empty

const obj = {};
console.log(!!obj); // always returns true, even if the object is empty

const totalKeys = Object.keys(obj).length; // returns the total number of keys in an object
console.log(totalKeys ? 'Not Empty' : 'Empty');
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7. Check if a property exists in an object

const obj = {
  test: undefined
};

// cannot differentiate if the property is not present or the value is undefined
console.log( obj.test ); // undefined

// the property exists
console.log( "test" in obj ); // true
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8. Loop over an object

const age = {
  john: 20,
  max: 43
};

// Solution 1 - Get 'keys' and loop over
const keys = Object.keys(age);
keys.forEach(key => age[key]++);

console.log(age); // { john: 21, max: 44 }

// Solution 2 - for..in loop
for(let key in age){
    age[key]++;
}

console.log(age); // { john: 22, max: 45 }
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9. Prevent an object's properties value from updating

const obj = {name: 'Codedrops'};
console.log(obj.name); // Codedrops

/* Set the 'writable' descriptor to false for the 'name' key  */
Object.defineProperty(obj, 'name', {
        writable: false
});

obj.name = 'ABC';
console.log(obj.name); // Codedrops
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10. Object keys are stored in insertion order

const obj = {
  name: "Human",
  age: 0,
  address: "Earth",
  profession: "Coder",
};

console.log(Object.keys(obj)); // name, age, address, profession
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Objects maintain the order in which the keys were created.


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