presidential-innovation-fellows/code-gov-web

import { Pipe, PipeTransform } from '@angular/core';

/**
 * Pipe used in boolean expressions in a template to signify
 * that the pipe value argument is defined using the
 * transform() method.
 *
 * If the transform() argument is defined, then the method
 * will return true. However, if the argument to transform()
 * is a string and it can be trimmed to an empty string,
 * then false is returned. Also, if the argument to tranform()
 * is a string with a trimmed value of 'null' or 'undefined;,
 * then false is returned.
 *
 */
@Pipe({
    name: 'isdefined'
})
export class IsDefinedPipe implements PipeTransform {
    /**
     * Test method used to implement PipeTransform
     * where the actual define check is done.
     *
     * @param value - The value to check if it is a defined value
     * @param args - Not used by this pipe
     * @return true if value is defined or if a string
     * that the trimmed value is not an empty string
     * or one whose value is 'null' or 'undefined', otherwise false.
     */
    transform(value: any, args?: any[]): any {
      if (typeof value === 'string') {
          return this.isDefinedString(value);
      } else if (typeof value === 'number') {
          return this.isDefinedNumber(value);
      } else if (value) {
          return true;
      }
      return false;
    }

    private isDefinedString(value: string): boolean {
        value = value.trim();
        if (value === 'null') {
            return false;
        } else if (value === 'undefined') {
            return false;
        } else {
            if (value) {
                return true;
            }
        }
    }

    private isDefinedNumber(value: number): boolean {
        // In this case, zero is truthy,
        //  since in JavaScript, zero is false/falsy
        if ( value === 0 ) {
            return true;
        } else {
            if (value) {
                return true;
            }
        }
    }
}