如何将值保存到 function 的变量中？

Question

This Code uses the Nativescript Plugin

let power = require("nativescript-powerinfo");

power.startPowerUpdates(function (Info) {
    console.log("battery charge: " + Info.percent + "%");
});

Console Log: battery charge: 100 %

I would like to save this Info.percent to a variable so I can later on reuse it.

Unlucky it always says, that its undefined. I tried different approaches.

Like:

var batterystatus = power.startPowerUpdates(function(Info){
      return Info.percent;
}

or

power.startPowerUpdates(function(Info){
          return Info.percent;

var batterystatus = power.startPowerUpdates(function);

or I also tried:

var batterystatus = power.startPowerUpdates(function(Info){
          this.batterystatus = Info.percent
}

But all pretty much deliver the wrong result.

typeof(Info.percent) = number

Answer 1

The method startPowerUpdates will call the function that you are passing to it, but it seems that startPowerUpdates doesn't return the same returned value. Either the function is called asynchronously or it simply wasn't programmed for it to return the same value. Try this instead:

let power = require("nativescript-powerinfo");

let batteryPercent;

power.startPowerUpdates(function (Info) {
    batteryPercent = Info.percent;
    console.log("battery charge: " + batteryPercent + "%");
});

Then check afterward if batteryPercent is set. If it is not then the function is being called in parallel and you'll have to code a trigger to let the program know that the value has been set. Example:

First check if the function is called synchronously (probably not):

...

console.log(batteryPercent); // check the value here

If the console log is undefined, then you'll need another approach:

let updateBatteryPercent = (newBatteryPercent) => {
    batteryPercent = newBatteryPercent;
    continueExecutionFunction();
}

// and now you should have:
power.startPowerUpdates(function (Info) {
    console.log("battery charge: " + Info.percent+ "%");
    updateBatteryPercent(Info.percent);
});

function continueExecutionFunction() {
    // here you should have whatever you'd like to
    // happen after you find out the battery percent
}

Ideally, you should use await and async functions, but that's more advanced. Don't play with them until problems like the one you had are easy.