Detailed introduction to the use of expressions in Vue.js

Vue.js is a front-end MVVM framework, where expressions are an important concept provided by Vue.js in templates. Expressions are mainly used to process data output and operations in templates to show the results expected by users. This article will introduce in detail how to use expressions in Vue.js.

What is an expression

In Vue.js, an expression is a JavaScript expression enclosed in double curly braces {{}}. Vue.js performs data binding and rendering by evaluating expressions.

For example, if we want to output the value of a variable message in the template, we can use the following code in the HTML file:

<div>{{ message }}</div>

Here{{ message }} is an expression, which will be replaced by the value of the variable message.

Of course, expressions are not limited to simple variable output. Expressions can also perform some simple operations, such as calculations, function calls, etc. For example:

<div>{{ message.toUpperCase() }}</div>

The expression here converts the value of message to uppercase letters and then outputs it to the template.

Restrictions on expressions

When using expressions, Vue.js has some restrictions:

1. You cannot use some JavaScript operations with side effects, such as using console.log and window.alert etc. for debugging.
2. Expressions can only be used to calculate values, and cannot be used for operations such as process control, looping, and exception handling.
3. ES6 syntax, such as arrow functions and template strings, cannot be used in expressions.

These limitations are due to the fact that the expression evaluation process of Vue.js is implemented based on JavaScript's eval function. In order to ensure security and performance, Vue.js evaluates expressions A series of restrictions were made.

How to use common expressions

In addition to variable output and simple calculations, Vue.js expressions also have some common uses. Here are a few:

Conditions Expression

In templates, we often need to decide what content to display based on a certain condition. In this case, conditional expressions can be used. Vue.js’ conditional expressions are similar to JavaScript’s conditional operator ?.

The following code will display different content based on the value of isShow:

<div>{{ isShow ? '显示内容' : '隐藏内容' }}</div>

List display

There is a special instruction in Vue.js v-for, can be used to loop through arrays and objects. In loop traversal, we can use expressions to get the value of an element and operate on the element.

For example, the following code can be used to loop through a list and output the value of each element:

<ul>
  <li v-for="item in list">{{ item }}</li>
</ul>

Function call

In an expression, we can call a method or Function, by accessing the methods in the Vue instance, you can handle some complex business logic.

For example:

<div>{{ formatDate(date) }}</div>

Here formatDate is a method in a Vue instance that can format time into a specified string.

Filter

There is also a very commonly used feature in Vue.js: filter. Filter is a function that can format data when output, and can be used to process data that needs to be formatted.

For example, the value of message can be converted to uppercase characters and intercepted in the following way:

<div>{{ message | uppercase | limit(10) }}</div>

where, uppercase and limit are all custom filters.

Summary

In Vue.js, expressions are a very important concept that can be used to handle data output and operations in templates. When using expressions, you need to pay attention to the limitations of Vue.js. You cannot perform some JavaScript operations with side effects, nor can you perform operations such as flow control, looping, and exception handling. In addition to simple variable output and calculation, Vue.js expressions can also use functions such as conditional expressions, list display, function calls, and filters.

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