node_modules/handlebars/lib/handlebars/helpers/if.js

import { isEmpty, isFunction } from '../utils';
import Exception from '../exception';

export default function(instance) {
  instance.registerHelper('if', function(conditional, options) {
    if (arguments.length != 2) {
      throw new Exception('#if requires exactly one argument');
    }
    if (isFunction(conditional)) {
      conditional = conditional.call(this);
    }

    // Default behavior is to render the positive path if the value is truthy and not empty.
    // The `includeZero` option may be set to treat the condtional as purely not empty based on the
    // behavior of isEmpty. Effectively this determines if 0 is handled by the positive path or negative.
    if ((!options.hash.includeZero && !conditional) || isEmpty(conditional)) {
      return options.inverse(this);
    } else {
      return options.fn(this);
    }
  });

  instance.registerHelper('unless', function(conditional, options) {
    if (arguments.length != 2) {
      throw new Exception('#unless requires exactly one argument');
    }
    return instance.helpers['if'].call(this, conditional, {
      fn: options.inverse,
      inverse: options.fn,
      hash: options.hash
    });
  });
}