# JavaScript Support

# Operational limitations

For security reasons, dynamic execution is not supported in Mini Programs JS Code, namely:

• Does not support the use of eval implement JS code
• Does not support the use of new Function Create function
  • new Function('return this') Except

# standard ECMAScript Support

Mini Program. JS Implementation environment Different execution environments on different platforms, thus resulting in different platforms' interest in ECMAScript There are differences in support of standards.

The Mini Program base library in order to smooth out these differences as much as possible, built in a core-js Polyfill。core-js The missing criteria for the platform environment API Make up.

It is important to note that the platform for ECMAScript Syntax support differences cannot be smoothed out when you need to use some advanced syntax, such as async/await When you need help. [Code Conversion Tool](https://developers.weixin.qq.com/miniprogram/dev/Devtools /codecompile.html#es6-%E8%BD%AC-es5) To support these grammars.

# Unable to be Polyfill of API

Following API Not available in some of the lower versions of the client, be careful to avoid using

• Proxy object

# Differences with standards

# Promise Timing difference

Because iOS JavaScriptCore Limitations, iOS 15 And the following Promise Is one that uses the setTimeout Analog Polyfill。Which means Promise The triggered task is a normal task, not a micro-task, which results in in iOS15 And the following Promise The timing will differ from the standard。

iOS 16 And above there is no difference.

var arr = []

setTimeout (() => arr.push(6), 0)
arr.push(1)
const p = new Promise(resolve => {
  arr.push(2)
  resolve()
})
arr.push(3)
p.then(() => arr.push(5))
arr.push(4)
setTimeout (() => arr.push(7), 0)

setTimeout (() => {
  // Should be output [1,2,3,4,5,6,7]
  // in iOS15 Mini Program environment, here will output [1,2,3,4,6,5,7]
  console.log(arr)
}, 1000)

The difference between a normal task and a microtask can be viewedThis article

# How to determine what is needed in the current environment Polyfill & Code conversion target

Specific mini program base library versions have minimum WeChat client version requirements, such as base library v2.15.0 Minimum Android version required 7.0.22，iOS Minimum version 7.0.20。

Specific client versions have minimum operating system version requirements, such as iOS 7.0.20 Minimum requirements iOS 10。

Thus, when a specific Mini Program base library version is specified (which can be found in Mini Program management page [Settings] - [Basic Settings] - [Base Library Lowest Version Settings]), we are able to get the minimum required support for the execution environment.

Specific data can be obtained from [This open source library](https://github.com/WeChat mini-program /miniprogram-compact ) Obtained in.