// Arrow function

reduce((accumulator, currentValue) => { ... } )

reduce((accumulator, currentValue, index) => { ... } )

reduce((accumulator, currentValue, index, array) => { ... } )

reduce((accumulator, currentValue, index, array) => { ... }, initialValue)

​

// Callback function

reduce(callbackFn)

reduce(callbackFn, initialValue)

​

// Inline callback function

reduce(function callbackFn(accumulator, currentValue) { ... })

reduce(function callbackFn(accumulator, currentValue, index) { ... })

reduce(function callbackFn(accumulator, currentValue, index, array){ ... })

reduce(function callbackFn(accumulator, currentValue, index, array) { ... }, initialValue)

​

// Array.prototype.reduce()

​

const array1 = [1, 2, 3, 4];

const reducer = (accumulator, currentValue) => accumulator + currentValue;

​

// 1 + 2 + 3 + 4

console.log(array1.reduce(reducer));

// expected output: 10

​

// 5 + 1 + 2 + 3 + 4

console.log(array1.reduce(reducer, 5));

// expected output: 15

var sum = _.reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);

=> 6

​

const reverse = (string) => {

  return string

    .split('')

    .reverse()

    .join('')

}

​

const average = (array) => {

  const reducer = (sum, item) => {

    return sum + item

  }

​

  return array.reduce(reducer, 0) / array.length

}

​

module.exports = {

  reverse,

  average,

}

import jwt_decode from "jwt-decode";

const initState = {

  loading: false,

  signUpErrors: [],

  loginErrors: [],

  token: "",

  user: "",

};

​

const verifyToken = (token) => {

  const decodeToken = jwt_decode(token);

  const expireIn = new Date(decodeToken.exp * 1000);

  if (new Date() > expireIn) {

    localStorage.removeItem("MyToken");

  } else {

    return decodeToken;

  }

};

const token = localStorage.getItem("MyToken");

if (token) {

  const decoded = verifyToken(token);

  initState.token = token;

  const { userData } = decoded;

  initState.user = userData;

}

const AuthReducer = (state = initState, action) => {

  if (action.type === "SET_LOADER") {

    return { ...state, loading: true };

  } else if (action.type === "CLOSE_LOADER") {

    return { ...state, loading: false };

  } else if (action.type === "SIGNUP_ERRORS") {

    return { ...state, signUpErrors: action.signupError };

  } else if (action.type === "SET_TOKEN") {

    const decoded = verifyToken(action.token);

    const { userData } = decoded;

    return {

      ...state,

      token: action.token,

      user: userData,

      signUpErrors: "",

      loginErrors: "",

    };

  } else if (action.type === "LOGOUT") {

    return { ...state, token: "", user: "" };

  } else if (action.type === "LOGIN_ERRORS") {

    return { ...state, loginErrors: action.loginErrors };

  } else {

    return state;

  }

};

export default AuthReducer;

​

function createStore(reducer) {

  let state;

​

  function dispatch(action) {

    state = reducer(state, action);

    render();

  }

​

  function getState() {

    return state;

  }

​

  return {

    dispatch,

    getState,

  };

}

​

function reducer(state = { count: 0 }, action) {

  switch (action.type) {

    case "counter/increment":

      return { count: state.count + 1 };

​

    default:

      return state;

  }

}

​

function render() {

  let container = document.getElementById("container");

  container.textContent = store.getState().count;

}

​

let store = createStore(reducer); // createStore takes the reducer as an argument

store.dispatch({ type: "@@INIT" });

let button = document.getElementById("button");

​

button.addEventListener("click", () => {

  store.dispatch({ type: "counter/increment" });

});

const arr = [5, 7, 1, 8, 4];const sum = arr.reduce(function(accumulator, currentValue) {  return accumulator + currentValue;});// prints 25console.log(sum);