How to get device width and height in Ionic using TypeScript?

You can use the Platform information for this.

Internally the platform uses the window.innerWidth and window.innerHeight but

Using this method is preferred since the dimension is a cached value,
which reduces the chance of multiple and expensive DOM reads.

Ionic 4/5

Docs: https://ionicframework.com/docs/angular/platform#width-number

import { Component } from '@angular/core';
import { Platform } from '@ionic/angular';

@Component({...})
export MyApp {
  constructor(platform: Platform) {
    platform.ready().then(() => {
      console.log('Width: ' + platform.width());
      console.log('Height: ' + platform.height());
    });
  }
}

Ionic 2/3

Docs: https://ionicframework.com/docs/v3/api/platform/Platform/#width

import { Component } from '@angular/core';
import { Platform } from 'ionic-angular';

@Component({...})
export MyApp {
  constructor(platform: Platform) {
    platform.ready().then(() => {
      console.log('Width: ' + platform.width());
      console.log('Height: ' + platform.height());
    });
  }
}