Javascript Interview Prep Questions (Pt-3)

Javascript Basic Questions (Part-3)

1.What is function currying?

• Currying in Javascript transforms a function with multiple arguments into a nested series of functions, each taking a single argument.
• Currying helps you avoid passing the same variable multiple times, and it helps you create a higher order function.
• Currying is a transformation of functions that translates a function from callable as f(a, b, c) into callable as f(a)(b)(c)

function curry(f) {
  // curry(f) does the currying transform
  return function (a) {
    return function (b) {
      return f(a, b)
    }
  }
}
// usage
function sum(a, b) {
  return a + b
}
let curriedSum = curry(sum)

console.log(curriedSum(1)(2)) // 3

2. What is Unmounting and Mounting? (React Lifecycle)

• There are 4 stages in a react lifecycle.

initialization

• This is the phase in which the component is going to start its journey by setting up the state and the props. This is usually done inside the constructor method.

mounting

• Mounting is the phase in which our React component mounts on the DOM.
• There are 2 methods in mounting. They are componentWillMount() and componentDidMount().

updating

• This is where component’s state changes and hence, re-rendering takes place.
• the data of the component (state & props) updates in response to user events like clicking, typing and so on. This results in the re-rendering of the component.
• There are 3 methods in updating. They are shouldComponentUpdate() , componentWillUpdate() and ComponentDidUpdate().

unmounting

• The component gets unmounted from the DOM in this phase.
• It has only one method and it is componentWillUnmount().

3. What are props?

• The props in React are the inputs to a component of React.
• The main purpose of props is to provide different component functionalities such as:

1. Passing custom data to the React component.
2. Using through this.props.reactProp inside render() method of the component
3. Triggering state changes.

4. What is a state?

• State of a component is an object that holds some information that may change over the lifetime of the component.
• The important point is whenever the state object changes, the component re-renders.
• It is always recommended to make our state as simple as possible and minimize the number of stateful components.

5. Difference between React state and props.

• In React, both state and props are plain JavaScript objects and used to manage the data of a component, but they are used in different ways and have different characteristics.
• state is managed by the component itself and can be updated using the setState() function. Unlike props, state can be modified by the component and is used to manage the internal state of the component.
• Changes in the state trigger a re-render of the component and its children.
• props (short for "properties") are passed to a component by its parent component and are read-only, meaning that they cannot be modified by the component itself.
• props can be used to configure the behavior of a component and to pass data between components.