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This is a subquestion related to Evaluation Symbol.
Is there something of the form \left<?> instead of \left. telling LaTeX to enclose the expression here and not to insert any delimiter at all, no matter wether visible or invisible?
As an example I want:
\left<no-delimiter><stuff>\right|_<index>
When \left. is used, TeX inserts no delimiter, but adds a space of width \nulldelimiterspace. It's not advisable to set this parameter to zero, in my opinion. But with
\newcommand{\noleft}{\left.\kern-\nulldelimiterspace}
using
\noleft x\right|_{a}
will in any case remove the added space.
As explained in my answer to the referenced question, using a macro would be much better for ensuring consistency. If your aim is to get code that works also with MathJax out of the box, you're on the wrong road: the capabilities of MathJax are limited and so you are forcing yourself into using bad LaTeX.
\usepackage{mleftright}
\[ x \mleft. \frac{1}{2} \mright|_i ]|
The package mleftright fixes the spacing of \left. and \right|, so that it is the same as in $ x \frac{1}{2} |_x $ but the | vertically stretches as you were using \left. and \right|.
So \mleft. should do what you do (null delimiter, no space), but consider that the space after \mright| is reduced too.
\left.but after setting\nulldelimiterspace=0ptIs that a correct interpretation of your question?.stand for spacing. Does there exist\nulldelimiter?\newcommand\noleft{\left.\kern-\nulldelimiterspace}