Error: Missing helper: "if_equal" handlebars

I tried to register handlebar on my express node application, but seems like it doesn't work

const express = require('express');
const hbs = require('hbs');
const expressHbs = require('express-handlebars');

const app = express();
app.engine('.hbs', expressHbs({ defaultLayout: 'layout', extname: '.hbs' }));
app.set('view engine', 'hbs');

hbs.registerHelper('if_equal', function(a, b, opts) {
    if (a == b) {
        return opts.fn(this)
    } else {
        return opts.inverse(this)
    }
});

In .hbs file I run this code

 {{#if_equal x "my_string"}}
       x is "my_string"
  {{else}}
       x isn't "my_string"
  {{/if_equal}}

Then I received this error

Error: Missing helper: "if_equal" handlebars