How to use the metal.core.isFunction function in metal

getSurfaceContent(surface) {
				var content = contentConfig[surface.surfaceElementId];
				return core.isFunction(content) ? content(this.component_) : content;
			}
		}

render() {
				super.render();
				if (core.isFunction(opt_rendererContentOrFn)) {
					opt_rendererContentOrFn();
				} else {
					this.component_.element.innerHTML = opt_rendererContentOrFn;
				}
			}
		}

callFunction_(fn, args) {
		if (core.isString(fn)) {
			return this[fn].apply(this, args);
		} else if (core.isFunction(fn)) {
			return fn.apply(this, args);
		}
	}

		setter: (val) => val ? (core.isFunction(val) ? val : () => val) : null
	},

renderTemplateByName_(templateComponentName, templateName, opt_data) {
		var elementTemplate = SoyTemplates.get(templateComponentName, templateName);
		if (core.isFunction(elementTemplate)) {
			return this.renderTemplate_(elementTemplate, opt_data);
		}
	}