javascript—为什么我们不能使用dom GetElementsByCassName选择的变量的addeventlistener？

oymdgrw7 于 2021-09-13 发布在 Java

关注(0)|答案(2)|浏览(326)

btn = document.getElementsByClassName('btn');
value = document.getElementById('value');

let counter = 0;

btn[2].addEventListener('click', function() {
  if (btn[2].classList.contains('decrease')) counter -= 1;
  if (btn[2].classList.contains('reset')) counter = 0;
  if (btn[2].classList.contains('increase')) counter += 1;
  value.innerText = counter;
});

/*
=============== 
Fonts
===============

* /

@import url("https://fonts.googleapis.com/css?family=Open+Sans|Roboto:400,700&display=swap");

/*
=============== 
Variables
===============

* /

:root {
  /* dark shades of primary color*/
  --clr-primary-1: hsl(205, 86%, 17%);
  --clr-primary-2: hsl(205, 77%, 27%);
  --clr-primary-3: hsl(205, 72%, 37%);
  --clr-primary-4: hsl(205, 63%, 48%);
  /* primary/main color */
  --clr-primary-5: hsl(205, 78%, 60%);
  /* lighter shades of primary color */
  --clr-primary-6: hsl(205, 89%, 70%);
  --clr-primary-7: hsl(205, 90%, 76%);
  --clr-primary-8: hsl(205, 86%, 81%);
  --clr-primary-9: hsl(205, 90%, 88%);
  --clr-primary-10: hsl(205, 100%, 96%);
  /* darkest grey - used for headings */
  --clr-grey-1: hsl(209, 61%, 16%);
  --clr-grey-2: hsl(211, 39%, 23%);
  --clr-grey-3: hsl(209, 34%, 30%);
  --clr-grey-4: hsl(209, 28%, 39%);
  /* grey used for paragraphs */
  --clr-grey-5: hsl(210, 22%, 49%);
  --clr-grey-6: hsl(209, 23%, 60%);
  --clr-grey-7: hsl(211, 27%, 70%);
  --clr-grey-8: hsl(210, 31%, 80%);
  --clr-grey-9: hsl(212, 33%, 89%);
  --clr-grey-10: hsl(210, 36%, 96%);
  --clr-white: #fff;
  --clr-red-dark: hsl(360, 67%, 44%);
  --clr-red-light: hsl(360, 71%, 66%);
  --clr-green-dark: hsl(125, 67%, 44%);
  --clr-green-light: hsl(125, 71%, 66%);
  --clr-black: #222;
  --ff-primary: "Roboto", sans-serif;
  --ff-secondary: "Open Sans", sans-serif;
  --transition: all 0.3s linear;
  --spacing: 0.1rem;
  --radius: 0.25rem;
  --light-shadow: 0 5px 15px rgba(0, 0, 0, 0.1);
  --dark-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
  --max-width: 1170px;
  --fixed-width: 620px;
}

/*
=============== 
Global Styles
===============

* /

* ,

::after,
::before {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: var(--ff-secondary);
  background: var(--clr-grey-10);
  color: var(--clr-grey-1);
  line-height: 1.5;
  font-size: 0.875rem;
}

ul {
  list-style-type: none;
}

a {
  text-decoration: none;
}

h1,
h2,
h3,
h4 {
  letter-spacing: var(--spacing);
  text-transform: capitalize;
  line-height: 1.25;
  margin-bottom: 0.75rem;
  font-family: var(--ff-primary);
}

h1 {
  font-size: 3rem;
}

h2 {
  font-size: 2rem;
}

h3 {
  font-size: 1.25rem;
}

h4 {
  font-size: 0.875rem;
}

p {
  margin-bottom: 1.25rem;
  color: var(--clr-grey-5);
}

@media screen and (min-width: 800px) {
  h1 {
    font-size: 4rem;
  }
  h2 {
    font-size: 2.5rem;
  }
  h3 {
    font-size: 1.75rem;
  }
  h4 {
    font-size: 1rem;
  }
  body {
    font-size: 1rem;
  }
  h1,
  h2,
  h3,
  h4 {
    line-height: 1;
  }
}

/*  global classes */

/* section */

.section {
  padding: 5rem 0;
}

.section-center {
  width: 90vw;
  margin: 0 auto;
  max-width: 1170px;
}

@media screen and (min-width: 992px) {
  .section-center {
    width: 95vw;
  }
}

main {
  min-height: 100vh;
  display: grid;
  place-items: center;
}

/*
=============== 
Counter
===============

* /

main {
  min-height: 100vh;
  display: grid;
  place-items: center;
}

.container {
  text-align: center;
}

# value {

  font-size: 6rem;
  font-weight: bold;
}

.btn {
  text-transform: uppercase;
  background: transparent;
  color: var(--clr-black);
  padding: 0.375rem 0.75rem;
  letter-spacing: var(--spacing);
  display: inline-block;
  transition: var(--transition);
  font-size: 0.875rem;
  border: 2px solid var(--clr-black);
  cursor: pointer;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.2);
  border-radius: var(--radius);
  margin: 0.5rem;
}

.btn:hover {
  color: var(--clr-white);
  background: var(--clr-black);
}

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Counter</title>

  <!-- styles -->
  <link rel="stylesheet" href="styles.css" />
</head>

<body>
  <main>
    <div class="container">
      <h1>
        counter
      </h1>
      <span id="value">0</span>
      <div class="button-container">
        <button class="btn decrease">decrease</button>
        <button class="btn reset">reset</button>
        <button class="btn increase">increase</button>
      </div>
    </div>
  </main>
  <!-- javascript -->
  <script src="app.js"></script>
</body>

</html>

当我单击数组中索引为2的递增按钮时 value 价值不会增加。

btn = document.getElementsByClassName('btn'); 

btn[2].addEventListener('click', function() {
  if (btn[2].classList.contains('decrease')) counter -= 1;
  if (btn[2].classList.contains('reset')) counter = 0;
  if (btn[2].classList.contains('decrease')) counter += 1;
  value.innerText = counter;
});

Arrays JavaScript Html css Dom

来源：https://stackoverflow.com/questions/68337496/why-we-cant-use-addeventlistener-of-variable-that-has-been-selected-by-dom-gete

2条答案

按热度按时间

ukdjmx9f1#

循环遍历节点列表，并向所有按钮添加事件侦听器。然后通过检查事件目标的 classList .

btn = document.getElementsByClassName('btn');
value = document.getElementById('value');

let counter = 0;

Array.from(btn).forEach((e) => {e.addEventListener('click', function(e) {
    if (e.target.classList.contains('decrease')) counter -= 1;
    if (e.target.classList.contains('reset')) counter = 0;
    if (e.target.classList.contains('increase')) counter += 1;
    value.innerText = counter;
  })
});

/*
=============== 
Fonts
===============

* /

@import url("https://fonts.googleapis.com/css?family=Open+Sans|Roboto:400,700&display=swap");

/*
=============== 
Variables
===============

* /

:root {
  /* dark shades of primary color*/
  --clr-primary-1: hsl(205, 86%, 17%);
  --clr-primary-2: hsl(205, 77%, 27%);
  --clr-primary-3: hsl(205, 72%, 37%);
  --clr-primary-4: hsl(205, 63%, 48%);
  /* primary/main color */
  --clr-primary-5: hsl(205, 78%, 60%);
  /* lighter shades of primary color */
  --clr-primary-6: hsl(205, 89%, 70%);
  --clr-primary-7: hsl(205, 90%, 76%);
  --clr-primary-8: hsl(205, 86%, 81%);
  --clr-primary-9: hsl(205, 90%, 88%);
  --clr-primary-10: hsl(205, 100%, 96%);
  /* darkest grey - used for headings */
  --clr-grey-1: hsl(209, 61%, 16%);
  --clr-grey-2: hsl(211, 39%, 23%);
  --clr-grey-3: hsl(209, 34%, 30%);
  --clr-grey-4: hsl(209, 28%, 39%);
  /* grey used for paragraphs */
  --clr-grey-5: hsl(210, 22%, 49%);
  --clr-grey-6: hsl(209, 23%, 60%);
  --clr-grey-7: hsl(211, 27%, 70%);
  --clr-grey-8: hsl(210, 31%, 80%);
  --clr-grey-9: hsl(212, 33%, 89%);
  --clr-grey-10: hsl(210, 36%, 96%);
  --clr-white: #fff;
  --clr-red-dark: hsl(360, 67%, 44%);
  --clr-red-light: hsl(360, 71%, 66%);
  --clr-green-dark: hsl(125, 67%, 44%);
  --clr-green-light: hsl(125, 71%, 66%);
  --clr-black: #222;
  --ff-primary: "Roboto", sans-serif;
  --ff-secondary: "Open Sans", sans-serif;
  --transition: all 0.3s linear;
  --spacing: 0.1rem;
  --radius: 0.25rem;
  --light-shadow: 0 5px 15px rgba(0, 0, 0, 0.1);
  --dark-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
  --max-width: 1170px;
  --fixed-width: 620px;
}

/*
=============== 
Global Styles
===============

* /

* ,

::after,
::before {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: var(--ff-secondary);
  background: var(--clr-grey-10);
  color: var(--clr-grey-1);
  line-height: 1.5;
  font-size: 0.875rem;
}

ul {
  list-style-type: none;
}

a {
  text-decoration: none;
}

h1,
h2,
h3,
h4 {
  letter-spacing: var(--spacing);
  text-transform: capitalize;
  line-height: 1.25;
  margin-bottom: 0.75rem;
  font-family: var(--ff-primary);
}

h1 {
  font-size: 3rem;
}

h2 {
  font-size: 2rem;
}

h3 {
  font-size: 1.25rem;
}

h4 {
  font-size: 0.875rem;
}

p {
  margin-bottom: 1.25rem;
  color: var(--clr-grey-5);
}

@media screen and (min-width: 800px) {
  h1 {
    font-size: 4rem;
  }
  h2 {
    font-size: 2.5rem;
  }
  h3 {
    font-size: 1.75rem;
  }
  h4 {
    font-size: 1rem;
  }
  body {
    font-size: 1rem;
  }
  h1,
  h2,
  h3,
  h4 {
    line-height: 1;
  }
}

/*  global classes */

/* section */

.section {
  padding: 5rem 0;
}

.section-center {
  width: 90vw;
  margin: 0 auto;
  max-width: 1170px;
}

@media screen and (min-width: 992px) {
  .section-center {
    width: 95vw;
  }
}

main {
  min-height: 100vh;
  display: grid;
  place-items: center;
}

/*
=============== 
Counter
===============

* /

main {
  min-height: 100vh;
  display: grid;
  place-items: center;
}

.container {
  text-align: center;
}

# value {

  font-size: 6rem;
  font-weight: bold;
}

.btn {
  text-transform: uppercase;
  background: transparent;
  color: var(--clr-black);
  padding: 0.375rem 0.75rem;
  letter-spacing: var(--spacing);
  display: inline-block;
  transition: var(--transition);
  font-size: 0.875rem;
  border: 2px solid var(--clr-black);
  cursor: pointer;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.2);
  border-radius: var(--radius);
  margin: 0.5rem;
}

.btn:hover {
  color: var(--clr-white);
  background: var(--clr-black);
}

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Counter</title>

  <!-- styles -->
  <link rel="stylesheet" href="styles.css" />
</head>

<body>
  <main>
    <div class="container">
      <h1>
        counter
      </h1>
      <span id="value">0</span>
      <div class="button-container">
        <button class="btn decrease">decrease</button>
        <button class="btn reset">reset</button>
        <button class="btn increase">increase</button>
      </div>
    </div>
  </main>
  <!-- javascript -->
  <script src="app.js"></script>
</body>

</html>

赞(0）回复(0）举报 2021-09-13

lndjwyie2#

首先，你的第三个字母有拼写错误 if 言出必行

if (btn[2].classList.contains('increase')) counter += 1;

而不是

if (btn[2].classList.contains('decrease')) counter += 1;

第二，更普遍的做法是：

let onClickUpdate = function() {
  if (this.classList.contains('decrease')) counter -= 1;
  if (this.classList.contains('reset')) counter = 0;
  if (this.classList.contains('increase')) counter += 1;
  value.innerText = counter;
};

//btn[0].addEventListener('click', onClickUpdate);
//btn[1].addEventListener('click', onClickUpdate);
btn[2].addEventListener('click', onClickUpdate);

赞(0）回复(0）举报 2021-09-13

我来回答

javascript—为什么我们不能使用dom GetElementsByCassName选择的变量的addeventlistener？

2条答案

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