How to get query parameters from URL in Angular 5?

Question

In Angular 5, the query params are accessed by subscribing to this.route.queryParams (note that later Angular versions recommend queryParamMap, see also other answers).

Example: /app?param1=hallo&param2=123

param1: string;
param2: string;
constructor(private route: ActivatedRoute) {
    console.log('Called Constructor');
    this.route.queryParams.subscribe(params => {
        this.param1 = params['param1'];
        this.param2 = params['param2'];
    });
}

whereas, the path variables are accessed by this.route.snapshot.params

Example: /param1/:param1/param2/:param2

param1: string;
param2: string;
constructor(private route: ActivatedRoute) {
    this.param1 = this.route.snapshot.params.param1;
    this.param2 = this.route.snapshot.params.param2;
}

This is the cleanest solution for me

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

export class MyComponent {
  constructor(
    private route: ActivatedRoute
  ) {}

  ngOnInit() {
    const firstParam: string = this.route.snapshot.queryParamMap.get('firstParamKey');
    const secondParam: string = this.route.snapshot.queryParamMap.get('secondParamKey');
  }
}

I know that OP asked for Angular 5 solution, but yet for all of you who stumbles upon this question for newer (6+) Angular versions. Citing the Docs, regarding ActivatedRoute.queryParams (which most of other answers are based on):

Two older properties are still available. They are less capable than their replacements, discouraged, and may be deprecated in a future Angular version.

params — An Observable that contains the required and optional parameters specific to the route. Use paramMap instead.

queryParams — An Observable that contains the query parameters available to all routes. Use queryParamMap instead.

According to the Docs, the simple way to get the query params would look like this:

constructor(private route: ActivatedRoute) { }

ngOnInit() {
    this.param1 = this.route.snapshot.paramMap.get('param1');
    this.param2 = this.route.snapshot.paramMap.get('param2');
}

For more advanced ways (e.g. advanced component re-usage) see this Docs chapter.

EDIT:

As it correctly stated in comments below, this answer is wrong - at least for the case specified by OP.

OP asks to get global query parameters (/app?param1=hallo&param2=123); in this case you should use queryParamMap (just like in @dapperdan1985 answer).

paramMap, on the other hand, is used on parameters specific to the route (e.g. /app/:param1/:param2, resulting in /app/hallo/123).

Thanks to @JasonRoyle and @daka for pointing it out.

You can also Use HttpParams, such as:

  getParamValueQueryString( paramName ) {
    const url = window.location.href;
    let paramValue;
    if (url.includes('?')) {
      const httpParams = new HttpParams({ fromString: url.split('?')[1] });
      paramValue = httpParams.get(paramName);
    }
    return paramValue;
  }