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javascriptstandardsfetch-apispecifications

Why fetch Body object can be read only once?

The fetch specification states that the readable stream Body contains the Body.bodyUsed flag which is initially set to false and then is set to true with a call of any parsing method.

Here's an example:

fetch('/some/path', (res) => {
    // res.body.bodyUsed === false
    res.json();
    // res.body.bodyUsed === true
});

If you try to call a method like res.json() or res.text() once again, an exception is thrown.

The question is: why that behavior is used? Why not to allow parsing that readable stream as many times as one wants? I found no explanation of the matter.

PS. In Chrome (and maybe other browsers), that flag is accessible as res.body.locked.

Solution

  • The question is: why that behavior is used? Why not to allow parsing that readable stream as many times as one wants?

    It is possible to read Response.body more than once by using Response.clone()