It converts Object to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true.

References:

• https://dev.to/macmacky/70-javascript-interview-questions-5gfi#16-what-does-the-operator-do
• https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript

var result = add(10, 20);
console.log(result); // 30

// Function declaration
function add(num1, num2) {
	return num1 + num2;
}
// Function expression
var add = function (num1, num2) {
	return num1 + num2;
};

Function declarations are hoisted to the top of the code by the browser before any code is executed. Specifically, all of the functions written with function declarations are “known” before any code is run. This allows you to call a function before you declare.

Function expressions, however, do not hoist. If you try to run a function before you’ve expressed it, you’ll get an error.

References

• Function expressions vs. function declarations
• Understanding Hoisting in JavaScript

React.PureComponent is similar to React.Component. The difference between them is that React.Component doesn’t implement shouldComponentUpdate(), but React.PureComponent implements it with a shallow prop and state comparison.

Function React component is that which accepts a single “props” (which stands for properties) object argument with data and returns a React element.

References:

• https://reactjs.org/docs/react-api.html#reactpurecomponent
• https://reactjs.org/docs/components-and-props.html

Currying is a technique of evaluating function with multiple arguments, into sequence of function with single argument. That is, when we turn a function call add(1,2,3) into curriedAdd(1)(2)(3) .

function curriedAdd(x) {
    return function(y) {
        return function(z) {
            return x + y + z;
        };
    }
}

curriedAdd(1)(2)(3)  // result would be 6

let curriedAdd_1_2 = curriedAdd(1)(2);
curriedAdd_1_2(3) // result would be 6
curriedAdd_1_2(10) // result would be 13

References

• Currying in JavaScript
• The Rise and Fall and Rise of Functional Programming
• Currying the callback, or the essence of futures…

If you have a large project with the need for flexibility and customization then Mocha is probably the choice for you. If you have a smaller project and don’t need the extra setup and configuration up front, Jest is probably the better option.

References

• https://blog.usejournal.com/jest-vs-mocha-whats-the-difference-235df75ffdf3
• https://medium.com/airbnb-engineering/unlocking-test-performance-migrating-from-mocha-to-jest-2796c508ec50