[the_imp]

In JavaScript, use https://www.npmjs.com/package/yaml for this:

    import assert from 'assert'
    import { parseDocument } from 'yaml'
    
    const flowDoc = parseDocument(`[1,2,3]`)
    flowDoc.add(4)
    assert(flowDoc.toString(), '[ 1, 2, 3, 4 ]\n')
    
    const blockDoc = parseDocument(`\
    - 1
    - 2
    - 3`)
    blockDoc.add(4)
    assert(
      blockDoc.toString(),
      `\
    - 1
    - 2
    - 3
    - 4
    `
    )