The returned value is always a string.
>

$\mathrm{String}\left(\right)$

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$\mathrm{String}\left(a\,b\right)$

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$\mathrm{String}\left(''1+1=''\,1+1\right)$

Onedimensional arrays of integer[1] are interpreted as strings.
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$\mathrm{hi}\u2254\mathrm{Array}\left(\left[72\,101\,108\,108\,111\right]\,\mathrm{datatype}=\mathrm{integer}\left[1\right]\right)$

${\mathrm{hi}}{\u2254}\left[\begin{array}{ccccc}{72}& {101}& {108}& {108}& {111}\end{array}\right]$
 (4) 
>

$\mathrm{String}\left(\mathrm{hi}\right)$

All other expressions are converted into strings.
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$\mathrm{String}\left(x+{y}^{2}\right)$

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$\mathrm{String}\left(''a''\,1..5\right)$

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$\mathrm{String}\left(f\left(\right)\,c\right)$

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$\mathrm{String}\left(\mathrm{hi}\,'',''\,\mathrm{world}!\right)$

${''Hello,\; world!''}$
 (9) 