Hydration Mismatch

A recurring problem when doing SSR are so-called hydration mismatches.

Hydration mismatch

We recommend having a clear understanding of what hydration is, see What is Hydration?

A hydration mismatch is when the content rendered to HTML in Node.js isn't the same than the content rendered in the browser.

Hydration mismatches can induce performance degradation and bugs and should be avoided.

Example

Let's imagine we render the current date's milliseconds:

<span>{ new Date().getMilliseconds() }</span>

This induces a hydration mismatch because the milliseconds rendered to HTML (e.g. <span>123</span>) won't match the milliseconds rendered on the browser-side (e.g. <span>167</span>).

To prevent the hydration mismatch, we should ensure that the same milliseconds value is rendered. We can do that by using the onBeforeRender() hook.

export { onBeforeRender }

async function onBeforeRender() {
  const milliseconds = new Date().getMilliseconds()
  return {
    pageContext: {
      milliseconds
    }
  }
}

<span>{ pageContext.milliseconds }</span>

What happens here is that pageContext.milliseconds will be set exactly once, so that the same milliseconds value will be rendered to HTML in Node.js and hydrated on the browser-side. (We use export passToClient = ['milliseconds'] to pass pageContext.milliseconds to the browser.)

Common causes & solutions

Common causes:

Rendered content is actually different. Make sure that your components render the same content in Node.js as well as in the browser.
Browser cache: While developing, make sure to disable your browser's cache to ensure that your browser doesn't load cached & outdated JavaScript.
Proxies: If you use a proxy, make sure it doesn't apply problematic HTML transfomers. For example, if you use Cloudflare, you have to disable Cloudflare's automatic HTML minifier.

With React:

Old React versions are buggy around stiching text segments leading to Warning: Text content did not match.. Make sure to use the latest React version.

Suppress Hydration Mismatch

There are situations when mismatches are inevitable. To suppress hydration mismatch warnings:

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