score:0

render() {
  let me = this;
  return (
    this.props.images.map(function(e, i){
      return <img alt={i} key={i} src={e} className="thumbnail" onClick={me.selectImage}/>
    })
  )
}

score:1

arr.map((e, i) => { /* 'this' refers to your component, not internal scope of the function */ })

score:4

const that = this;
return (
  this.props.images.map(function(e, i){
    return <img alt={i} key={i} src={e} className="thumbnail" onClick={that.selectImage}/>
  })
)

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